What is the Segunda Etapa? Has it ever been explained? It certainly can't be like a Shikai or Bankai as it is unsealing one's power so could it be like an actual powerup instead of taking off training weights?
I don't think it was ever explained. It seems to be suggested in some media that Ulquiorra was simply a rather special Hollow that allowed him to have a Segunda Etapa but nothing that was really confirmed.
Going by UNMASKED, Segunda Etapa is a form unique to Ulquiorra. Resurrección returns an Arrancar to their original Hollow form, whilst Segunda Etapa returns Ulquiorra to the original form of his tribe. Ulquiorra thought of it as the source of his emptiness and, for these reasons, didn't show it to Aizen. The above is explained in UNMASKED and the short story not be, but be.
Unfortunately, Ulquiorra's tribe (unnamed) is the only one I can think of. If you're interested in further reading, then I suggest you search online for the UNMASKED translations. Some real nice titbits in there.
Didn't mention Nnoitra's case because the group he killed was never specifically said to be a tribe. The term used in the manga is "colony", which is a very different thing. Another example is Grimmjow's Fraccion. They aren't a tribe either; merely a group of Arrancar who travelled and hunted together, no different to the Hollow Nnoitra killed.
I don't think standard rules can be applied to White. He's an experiment made by Aizen who wouldn't exist in the natural world otherwise. As for Segunda Etapa, I'm afraid I have to say that's a no. It's stated outright that Ulquiorra is the sole example.
"Tribe" and "colony" carry very different meanings. For one, a tribe is a group; one of their possible connections is blood-ties. A colony, meanwhile, is described as a group sharing nationality or race. Also, I didn't say Grimmjow and co. were a colony. I merely used them as an example of a group unrelated to each other that roamed Hueco Mundo together. Pack is a better term for them, though. Ulquiorra's tribe are specific in that they share physical characteristics such as a humanoid shape and a demonic appearance. The colony killed by Nnoitra where never shown, just mentioned, so we only have so much to go on. The above is merely how I interpret it.