i tossed 2 cory cats with my most aggresive boy in the same 5g tank, they lived happily ever after for about 3 months, until i decided to switch tanks for the cory cats,
and quite actually, we're still in belief that our male thought the cory cats were female betta, and was making a bubblenest to impress them.
ever since we took out the cory cats, he hasnt made a nest once, and just seems angrier at the world.
It comes down to personality with betta, thats all that matters.
its not suggested but you can hold up to 2 females and 1 male betta in the same 5g tank, atleast i could, had them together for about a week, (first roughly 10hours were supervised of course) and not a single nipped fin from any of it, or even a stress line on my most sensitive female. the male never even tried to make a nest, or anything.
(Granted i picked out my least aggresive male to see if it would be possible. and sure enough they coexisted peacefully.)
But dont get me wrong we had 2 clay pots in the 5g, and about 12 plants (Not over crowded with plants like your probably thinking) but enough to give many of hiding places for the females incase my male got rowdy.
I actually experienced more of an agressive behavior from the 2 females with each other, then i did with the male.
the 2 girls liked to flare at each other (gills and all) and just kinda look at each other, until one decides theyve had enough of the stare off, and she swims away.
Again not recomended, but possible if you feel you truely know your bettas personality enough to be able to tell how he/she would do in a community tank type of environment.