Sounds like your typical Black covict cichlids, they begin to get some red in the bellies (more so the female than male), they max out at about 4" and I highly reccomend not having them with anything communal or even Semi-agressive (unless the semi-agressive fish in question can hold it's own, such as RTBS's and Senegal Bichirs).
These fish are highly agressive when breeding and have been known to kill other fish if they come to close to the spawn pit, they are parental cichlids, meaning they will look after their young for a time.
As said, tank mates should be able to hold their own against these guys, they've been known to even give an oscar a good scare, Cichlid species should be their only companion (except Angelfish and Discus).
Hope this helps.