User blog comment:GinIchimaru/The Zero Squad: Reality vs Speculation?/@comment-1706259-20100413011856

"4. One of the Zero Squad members is the previous Squad 12 captain Hikifune" We know that she got promoted 110 years ago; but not if she is still part of the zero squad.

The fact that each member (the fact that we don't know how many of them are currently in the zero squad bothers me) has a bankai and is a captain level fighter does make them the strongest single group of shinigami.

Its possible that they keep themselves in shape by fighting against each other regularly; and so they would also make great allies in combat.