Dividers are more trouble than their worth in my opinion...plus they kind of look tacky and unnatural.
Personally when I want to keep several bettas, I get them each a small tank of their own.
My advice regarding a ten gallon tank would be to have one betta, and some other betta-friendly fish such as otos, corys, and (if the betta is mild-mannered) a small school of friendly tetras such as neons...or a school of harlequin rasboras or maybe white cloud minnows.
The variations are your choice, but the reasoning is this: some fish will harrass the betta and nip them into stress. These would include fiesty tetras such as black widows, serpaes and some others. Fiesty cyprinids like barbs and zebra danios will also nip at the betta.
Then there are the fish that even a mild mannered betta is likely to harrass such as the guppies and mollies. Whereas a mild-mannered betta will largely ignore the aforementioned schooling fish. He will almost definitely, regardless of temperment ignore the corys and otos.
Okay I'm sounding too disorganized here. I'll just summarize:
Nice betta can live with corys, otos, non-nippy tetras, harlequin rasboras and white cloud minnows.
Mean betta can live with corys and otos.
Really mean betta should be kept alone.
