Id tend to agree, buying a new fish, especially one as aggresive as a Firemouth and one of its size is something you shouldnt be doing. You should be rehoming all your current fish and then thinking about what you want before even thinking about buying.
A Firemout will take over the entire bottom of a 30 Gall tank without a doubt. If its in with the Rams they wont stand a chance.
Oh dear i had a group of firemouths of 3 firemouths in a 30g tank (as juvies) but one was very aggresive. He became such a bully i had to split up the group. They are lovely fish but a ram wouldn't stand a chance.