measure your tank, take those measurements down to the last calculator at the Tropical Chit Chat page, input the measurements (forget gravel, it's the same with or without) and go by that. a Betta is always 2 inches at minimum.
edit: a Betta will eat a Guppie thinking it is a young or runty male Betta. don't mix the two.
i use that as a guideline, i'm not one to believe in overstockage in established tanks. it's my opinion, it works well for me, i haven't lost fish yet, so don't bite my head off.