I have a betta in a 10 gallon with 2 corys and 6 neons -- They are all fine - Every once in awhile the betta chases the corys but they all do well.  Although I'd like more in that tank, I think its enough (and heavily planted) for a 10 gallon.  What you mentioned might fill my 20 gallon!  Careful -- if you overstocka nd too quickly you'll lose yor betta.