Bettas can live with almost any other fish of reasonable size that is not known for aggression. Mine is currently in a 55g with platies, SAEs, dwarf chain loaches, cardinals, a pearl gourami, a German blue ram, and congo tetras. Oh, and three female bettas. Danios should also be fine, or dwarf rainbows, or ember tetras. Female guppies would be fine, and usually male guppies are OK.
These are all OK if you have adequate space. Don't try a female betta as a companion in a 5 or 10g tank. In a heavily planted 30+, no problem. Thats 120 litres. The bigger the tank, the more of a natural environment your fish have and you will be rewarded with their natural behavior.
Tankmates to avoid are any of the aggressive gouramis, including dwarf gouramis, and any of the nippy tetras. And especially avoid tiger barbs. I saw someone put a betta into a tiger barb tank once and they swarmed the poor thing. Looked like sharks having a feeding frenzy. Luckily there was a net handy. Anything else nippy or aggressive should not be put in the betta tank. Pictus cats or red tailed sharks or rainbow sharks would probably get too aggressive as they got older.