i disagree with guppy... every betta i have kept (and i currently have eight of them!) has tolerated new tankmates very well... they usually do a little flaring while the new guys are floating, and once they are added to the tank, they are ignored completely. of course, it depends on who you're trying to add, but most of the fish you've mentioned shouldn't be a problem at all, unless you've got an insane attack betta.
Kuhlis get sort of long, require lots of hiding places, and like being in groups larger than a 5g can accomodate; so i would suggest against them.
a pair of corys or four neons should do just fine. maybe a few little shrimp. or the frogs.