I don't even want to know how all the fish you have can fit in a 100g. I also don't want to know what you are going to do once they are all full grown.
As for the firemouth, it is hit or miss as it is with any cichlid. Most sources say they are peaceful but mine is a little jackass and trys to kill everything that enters the tank except for the 4 Buenos Aires tetra. My advice to you however...don't get one, you should be saving money for more tanks or getting rid of more fish.