I made the mistake once of underjudging the aggressiveness of a convict cichlid and put one in a 30g with a firemouth and a severum. After realizing that the convict needed to be by itself I put a tank divider in (i had no room for another tank) and kept the firemouth and the severum on one side. I put a lava rock with a cave in it and the firemouth seemed content with just sitting inside the rock and never coming out. When it did squabble with the severum, it was never over aggressive, just kinda chased it away. It got to be about 4" and lived its life inside a lava rock without any apparent stress. So if you get a firemouth and are worried about it being too aggressive, you might wanna try making a cave, either with a lava rock or an upside down flowerpot and it might just sit there and not bother the other fish...