User blog comment:RoyalGuard-Elite/Isshin's Shikai/@comment-4509522-20131014173201/@comment-10579249-20131016174715

I wouldn't be so sure. Personally, I don't believe that Getsuga Tensho is a shikai ability (and technically it isn't seeing as Ichigo used it without his actual Zanpakuto). So it makes sense that Isshin could use as an attack, rather than an ability. Honestly, I think it's possible for any shinigami to use it, and Kenpachi actually comes very close to using a Getsuga Tensho with his "super slash" or whatever it is that releases massive amounts of spiritual pressure and power in a single strike. So i doubt that the type of Zanpakuto has any impact on that particular attack.