Size matters here. For every cichlid pair, you need roughly 24"x24" territorial space for them to defend. If you have a 6 or 8 foot long tank than you are fine, but if you have a 40 breeder with 6 firemouths and one pair decides to breed you will have trouble.
It's also about sq.ft. perimeters, not so much about how deep the water is. The tank can be 12 inches deep or 48 inches deep, but if it is 48" deep with a 2'x2' footprint it will still be a problem. It is way better to have a wider and longer tank than it is to have a deeper one. So a gallon-age rule is not always applicable.
The new Marineland wide tanks that are 36" front to back are ideal for many cichlids. There 300g Deep Dimension tank is awesome!
http://www.marineland.com/sites/Marineland/products/productdetail.aspx?id=2054&cid=3600&mid=3228
I keep my FM's in a 180g so a group of about 14 do fine.