I think you would be fine with a single mated pair of multies Neolamprologus multifasciatus, maybe a trio, but you would need to remove the inevitable fry. They are very tolerant of fry and will raise them up in the 7 gal provided you have plenty of shells, but as they mature you will need to remove them. The young grow slowly and you would have plenty of time, it's not like they will need to be removed in a matter of weeks, or even a few months.