I would put no more than 4 fish total in a 29 gallon. My personal preference is a pair or you risk aggression as the clowns mature. A harem or trio can be attempted but you need to start with young clowns not of sexual maturity yet.
depends on what clowns. put a maroon or worse, a pair in that tank and they will cause all kinds of problems. I would only go with one clown. I agree that four is probably ok. dartfish, firefish, blennies, gobies, would all be fine.