Firemouths are a shot as long as niether of them are breeding, I'm currently keeping them with mine, generally what you will find with compatibility is that if they are two fish in a tank, one is a bottom dweller, one a mid-region then they won't be much conflict. I have learned this from personal experience, angels can be rather aggresive. Mid-regions tend to occasionally nip top region swimmers (dianos are a good example) angels are mid-water. Basically your looking for any tempermental or semi-aggresive bottom dweller (NO AGGRESIVE) for instance don't choose a red tail shark because they can be very aggresive and sometimes swim midwater, or on the other hand your looking for a tough but peaceful mid-water fish, but with angels I wouldn't reccomend any top-region fish