Bettas are very hit-and-miss in community tanks. However you could look into small corys like panda corys or pygmy corys, also check out otocinclus.
You could try neons or harlequin rasboras but just make sure you have a back up plan for the betta if he becomes too aggressive.
You can put tetras in there. Try to avoid diamond tetras because bettas do not like them much. You can out neon tetras with them, golden tetras, blood fin tetras (I think that is what they are called.) Just about any tetra is fine but not diamond ones.
Betas are really that aggressive? Almost no other fish can be with one?