I've never had con's but i do currently have a pair of firemouths, and they are very very aggressive in my experience. I had a pair of them with a pair of juvenile oscars, all introduced together at the same size, and the firemouths had to be removed because I was scared that they'd kill the oscars. There were plenty of hiding spaces and plenty of room, the firemouths were constantly chasing the oscars, all day and night.
I now have them in a 37g which seems to be plenty of room, I plan on adding a few others to that 37, but i'm in no hurry.