Betta could have died of other causes and the other fish might of picked at him, shredding his fins.

I'd take a sample of water to a pet or fish store that does free water testing just to make sure everying is ok (checking levels of ammonia, nitrite, nitrate) in the tank. Seems like a lot of fish have made it into that tank in short amount of time so i'd check, just to be safe, that your not going into a mini-cycle (or still in the middle of the initial cycling process).

Plecos like to hide out under things, just dip your hand in and move stuff around to see where he's hiding. Also check behind your tank just in case.