Isshin couldn't have been 5th because Gin was his VC for a while who got promoted to 3rd Captain. Now Isshin could either have been 3rd or 11th. We can't rule out the 11th yet because if you look at what looks like what is left of a Captain's robe on Isshin in Ch 188, it looks very similar to Kenpachi's with the pattern and tears along the bottom. It's frayed and has black dots along the bottom. 11th would seem to fit his personality the best.