It depends largely on your betta's personality, some are more agressive than others. If your guppy has a fancy tail, the betta is likely to attack it, although some bettas are laid back and won't, and he may or may not attack or kill the other small fish. The tetras may nip at his fins which could stress him and lead to fin rot, that sort of stuff. I don't know if harlequins are nippy. Keep an eye on the tank and be prepared to separate him if needed, but he may adapt well, it's hard to say with bettas.