Agree with essjay, please do not keep a Betta in a community tank. But to comment on your other question, about danio and platy, just want to point out that the danio are shoaling fish, meaning they live in very large groups so to be settled and not stressed they need a group of their own. Minimum numbers are a bit misleading, as shoaling fish will always be better the more there are, but you do not want fewer than six or seven of a species of danio. [Tetras, barbs, rasbora and many catfish like cories are similar, being shoaling fish needing groups of the species.]