What kind of fish co-exist with bettas peacefully?

Bettas are usually only agressive amongst themselves and other fish with similar long flowing fins such as guppies and angelfish. The best tankmates are peaceful dither fish such as neons, cardinals, glowlights and other small, peaceful tetras. You can also choose harlequin rasbora and the bettas' closest relatives, the anabantids. These include the various gourami, but for your tank size, choose the smaller dwarf gouramis as they only grow up to 2 inches I think. As for the algae eaters and substrate dwelling fish, try to include a couple of ottos or siamese algae eaters and a small school of cory cats. Follow the inch per gallon rule because water quality can deteriorate if especially such a small tank will become crowded. Hope you enjoy your tank and keep up with the water changes. Best of luck!