Fossorochromis are usually peacefull untill they get around 10". After that they will be so dominant that they give the other fish a hard time.
Its not that the Fossorochromis will kill the other fish in the tank, but they don't accept other dominant males in the tank(except if the male can fight back. Think of a Buccochromis, Nimbochromis Fusco, Tyrannochromis etc).
If given enough time, proper feeding and regular tank cleaning, these fish grow to 35cm(about 14"). Expect the male to be at this size at 4-5 years. They grow pretty quick to 10-11" after that its a slow steady growth to the maximum size.