if you are talking about the african cichlid shell dwellers then yes, you can put them in a 30 gal
since most are fairly small cichlids.
Your options will be limited as far as the mbuna. You could do a trio of yellow labs, rusties, or saulosis. I would say around 6 or 7 fish max in a tank that size. Stay away from the johanniis (if you are talking about melanochromis johannii) they are far too aggressive for a tank that size.