I heard about diferent feeding problems with AR rifles and types of ammo. Some cheaper AR models have trouble cycling certain types of ammo, some of the higher end models will eat anything. Some ammo won't feed right at all in them.
My advice... find a few particular models you're considering whether it be seeing them at places like Four Seasons (Bushmaster dealer) or a few for sale online, at shops, etc. Go on the net and do some research on them. Check AR forums and ask questions on reliability, ammo, etc. and study results. An AR rifle is a big investment for most of us, and I for one wouldn't be buying one blind w/out getting as much info as possible first. I'd say you're off to a good start in asking about ammo, now go further.
BTW, I have the same model Kel Tec rifle and love mine. I noticed you put a butt bad on yours. I'm guessing for length of pull, something the Kel Tec Sub 2000 lacks big time. Is that from Kel Tec, or aftermarket made to fit?
I've been thinking of getting a top rail scope mount for mine with a red dot sight system. Kel Tec sell sthem directly.