The Espada (and other Arrancar) aren’t really intended to be an army - they’re research subjects that Aizen was basing his own Arrancar metamorphasis off.
The first four Espada were all naturally occurring partial Arrancar - that’s why Aizen went out of his way to obtain them. He pieced together how to make a completed Arrancar from their partial attempts.
The other six Espada (and Privirons etc) are test subjects exploring what can be obtained through the process of Arrancarnification (or whatever word you want to invent for the process of becoming an Arrancar). He was fine tuning what abilities he wanted to apply to himself.
(Side note: I'll try to point out what Aizen applies to himself from each Espada)
Espada 1 to 4 Vasto Lorde (Partial Arrancar);
(1) Coyote Starrk & Lilnette Gingerbuck - The original Vasto Lorde separated out it’s power from it’s main body and then contained it in a separate vessel, resulting in two partial Arrancar entities, Starrk & Lilnette.
(Side note: Aizen probably got his ability to fire multiple cero simultaneously from Starrk).
(2) Barragan - When he evolved to Vasto Lorde he formed a psuedo-zanpaktou in the form of his battleaxe. This is part of his power housed in a spirit weapon. (Note, his bizarre skeleton form means he isn’t unmasked at all).
(Side note: The lining of Aizen’s robe changes from black to Barragan’s purple when he starts to Arrancar, though this might be anime only).
(3) Harribel - Aizen has stated that he believes the truest form of Bankai to be when the user becomes one with their Zanpaktou and clads their power on/in their body - as seen in Ichigo, Hitsugiya, Yamamoto, Kenpachi and Bakyuya’s final Bankai form.
Harribel is partially unmasked and her hollowfication has taken on the form of a large blade - she basically looks like she’s in true!Bankai. She’s the closest to being a complete Arrancar. (She’s also possesses the reasoning capacity of a plus soul. No point unlocking loads of power if you lose your capacity to reason).
(Side note: I can’t figure out what Aizen applied to his own Arrancarnification off Harribel).
(4) Ulquiorra - If you’ve read his backstory from Bleach Databook Unmasked then you know that Ulquiorra became a partial Arrancar when he accidentally broke his mask. Ulquiorra is the an example of stable Arrancar that occurred due to external forces instead of occurring as part of a major evolutionary change.
(Side note, Aizen’s chrysalis stage appears to be based off Ulquiorra’s Vasto Lorde form. And his form after that look like an Aizen/Ulquiorra 1st release/Grimmjow mash-up - all-in-one glove-sleeved long coat (off Ulquiorra) with body suit underneath with the v-neck and large collar + that hair (off Grimmjow)).
Put all four of them together and you get the completed artificial Arrancarnification process - breaking a hollow mask (4) to separate out power (1) into a spirit weapon (2) to create a broken mask and blade form (3).
Espada 5 - 10 (& Priverons) are various trials, experiments etc.
Most of these Arrancar are Adjuchas who have been modified I.e. they have had certain aspects enhanced to Vasto Lorde level. When analysing battle prowess Tite used a hexagon chart to display Reiatsu (Kido), Offense (Zanjutsu), Defence, Mobility (Hoho), Physical Strength (Hakuda) and Intelligence on a scale 0 - 100. The idea behind modification is subtracting from one of these six aspect to enhance another aspect or create/enhance a unique ability. This is why these some Arrancar are great in a couple of things, but have glaring, crippling gaps in their abilities otherwise.
(Reminder - The four main combat abilities for Shinigami are; Kidō (Spells/energy attacks), Hakuda (Hand-to-hand - strength, flexibility, reflexes), Hohō (Fast Step - mobility) and Zanjutsu (Swordsmanship/Wielding Spirit Weapons - how well you can cut). Hollow/Arrancar abilities; Hiero (Steel Skin), Cero, increasing rietsu by consuming other spirit bodies, housing special/unique abilities within their body. Both Hollows and Shinigami can increase reiatsu through increased endurance, combat etc).
(5) Nnoitra - Enhanced hiero, enhanced physical strength and enhanced Zanjutsu. No kido-like abilities (can’t even block such attacks), is pretty slow and inflexible, no ranged attacks (excluding his chained Zanpaktou), no energy based attacks beyond cero. His reflexes don’t seem that sharp either. He’s enhanced to be a melee spirit weapon fighter, however, anything outside of that and he’s in trouble…
(Side note - Nel, whilst a similar type, is better all round except for Hiero. Aizen lost because his strength wasn’t quite as strong as Ichigo. I like to believe that Nnoitra’s machinations in getting rid of Nel (the superior template) fucked Aizen over ;D)
(6) Grimmjow isn’t modified. Aizen was testing what effect Arrancarnification has on someone who is naturally an overpowered anomaly. Monitoring Grimmjow’s growth - and pitching him against Ichigo to fuel his growth - are probably the only reasons Aizen didn’t just dispose of him for his rebellious attitude. Grimmjow’s speciality is Hakuda, then Hoho anyway, and his energy based / kido-like abilities are pretty good, and he can definitely cut. (He’s built similar to Yoruichi and who’d mess that up? Felines FTW).
(With Menos the more compact and closer to human size, the stronger the Menos is. It also seems to be the case that the less superfluous Hollow-growths the stronger the individual. Grimmjow was sleek and tiny, indicating he was about ten (?) times more powerful than an Adjuchas has any right to be, likely putting him at the power level of a Vasto Lorde. This is somewhat guess work - Tite has a thing for power increasing by a factor of ten and the only straight up Vasto Lorde depicted is Ulquiorra in his Unmasked backstory, as Espada 1-4 were partial Arrancars and thus more powerful than a standard Vasto Lorde. Presumably, if Grimmjow lives long enough, fights hard and eats well, he’ll evolve into the stuff of shinigami nightmares).
(Side note; Aizen broken mask after being defeated and powering down from his released state mimics Grimmjow’s jawbone mask. That part of his face remains sealed away during Thousand Year War, so I wonder if he’s kept it. He is now an Arrancar, so he should have a broken mask. Also, he totes got Grimmjow’s power mullet).
(7) Zommari - Enhanced Kido-like abilities and enhanced Hoho-like abilities. Poor physical attacks, his Kido and Hoho were incompatible - he could use one or the other.
(Anyone else notice he has the same power as Pepe? Poor Byakuya had to deal with controlling ‘love’ freaks twice! Throw in Tsukishima trying to control him by being his ‘life-long BBF’ and that three times. WTH?)
(Side note, Aizen snaps his neck when he evolve to final form in a similar manner to Zommari’s zanpaktou release. Because you totally wanted to see that horror show twice, right?)
(8) Szayelaporro - Rapid energy and ability analysis and countering, enhanced Kido-like abilities, plus his other nightmare fuel modifications. It’s hard to know what was Aizen and what was Szayelaporro, other than those he’s stated he did to himself. Szayelaporro is modified to a ridiculous level, so who knows what Aizen took from this. My guess is enhanced ability to rapidly analyse and form direct countermeasure to reiatsu based attacks. Aizen could do that anyway, but enhancing that ability would be to his benefit.
(Side note, pushing this a bit, but Aizen could have gotten ‘wings’ with things coming out of them from Szayelaporro. Y/N?)
(9) Aaroneiro - The ability to gain other spirits’ abilities by consuming them. If it weren’t for the fact that Aaroneiro was unable to evolve from Gillian he’d be more of a threat. As it was he was just a bloated mess who couldn’t reconcile his gains. Thoough it seems Aizen might have still appreciated his enhancement, just smarter about his 'eating habits'. Aizen did say when he was done he was going to ‘eat’ Ichigo.
(Side note: Aizen does end up looking like an Eldritch abomination. Just saying…)
(10) Yammi - Unique ability to continuously generate and grow in reiatsu without needing to consume any reiatsu or train/fight up his endurance/stamina etc. He was pretty much trash otherwise - no kido, slow, poor defense, weak for his size etc. It seemed most of his abilities (and intellect?) were traded in for this one trait. Aizen is clearly reaping the benefits of this research, as even sealed to his teeth he’s still growing in power.
(Side note: Aizen got bigger? Kidding. Yammi could just keep throwing out different release forms, which may be Aizen's main take away. I imagine if he escaped after the time skip his release form would have changed to account for his reiatsu growth).
(Former 3) Neliel - Enhanced speed, enhanced physical strength, enhanced Zanjutsu, healing abilities. Can absorb, combine and reflect energy attacks. No kido-like abilities (can’t even block such attacks), affected intellect (?) Her body shape would indicate javelin and charging attacks are her mainstay, so poor flexibility, hand-to-hand etc. Her defense wasn’t great either.
Aizen does NOT demonstrate Nel’s special abilities. He can’t absorb energy attacks. This indicates that he did base his own Arrancar metamorphosis off just the current Espada, not historical data. Adding Nel to his matrix would have probably made things turn out differently, for he could have then absorbed Ichigo’s energy attacks. Seriously, Nnoitra screwed Aizen's plans up. The number listing system was supposed to create a competition where the best Arrancars of each 'type' won, thus Aizen would use them as his base, but Nnoitra cheated.
Sorry, long post. Thoughts? Inaccuracies? Alternate suggestions?