I've been told to stay away from colorful or long fined fish with betta community tanks because it can lead to nipping and/or the betta getting territorial over what it thinks is another betta.
you could get five female bettas and do a sorrority. or divide it in half and have 2 boys.
or, if you wanna have one male these are some acceptable tankmates:
shrimp(they may get eaten, but ghost shrimp are cheap and easy to try out as a tank mate).
snails(apple or Olive Nerite have been suggested)
African Dwarf Frogs(not the african clawed frogs)
Otoclinus Catfish(alot ofpeoplelike having oto's, active and like to be in groups)
Corydoras Catfish(come in a few colors, like albino or panda, must get 4 or more..but can be a heavy bioload, over filtering a tengallon is recomended but idk what that means..)
....basicly bottom feeders xD
but White Cloud Mountain Minnows are peaceful with bettas ifyou want a non-bottom feeder
The tetras are kind of a toss up, some have had success and others haven't, you'd need 6 of them.
Platies are a no-no.
this is all a summed version of what i found here: http://www.ultimatebettas.com/index.php?showtopic=14389
a good site full of people who have alot of experiance with bettas. They list other fish you can't pair with, and a couple who are maybies.