Eric,
I didn't know u100's and the conversion chart were an option until I was just about done with the insulin for Bob. Otherwise, I would definitely have used them. When you get down to .25 units per shot, they sure come in handy.
Unless your kitty is long haired, the shorter the needle the better, so I would go with the 5/16ths length. (even if kitty is long haired, you can use them. Just harder to see the skin unless you wet the hair around the shot site first) As far as gauge, go with the higher number. I used 29 gauge, since that is what I had, but if you can get 31's go with that. 3/10ml is pretty much the standard, but that doesn't really come into play at the doses most cats get. More important for humans, as it determines the most units that the syringe will hold, which is important if you are shooting 30 or more units a day. Won't matter for you or anyone else here unless they have an Acro-kitty.