Some thoughts on this.
First Firemouths tend to be mellower than some, but as others have shared with you individual personalities are very broad spread with this species. That said, right now your lone Firemouth may be running scared. But give it time to grow and the story can change.
And contrary to what someone told you on the first page of this threat one firemouth is not necessarily more safe than a group. Generally when you have several firemouths in a tank they will often negate each other's aggression except for whatever dominant pair(s) control their group.
Next thing, Tiger barbs are strong fin nippers and a bad idea for angelfish. In a large set up even cardinal tetras eventually risk becoming food as angelfish grow larger. The rest such as black widow/black skirt tetras are fine.
Can you keep angels and firemouths together in a tank? Sure.
But if you do so you will need plants potted in planters that with large enough stones on top to prevent the firemouths from digging them up. Avoid sharp stones such as lace rock or tufa as skittish firemouths can cut themselves on such rockwork. Driftwood is better. Large groups of firemouths will basically stifle much breeding and keep aggression predominantly among their own kind.
In the end you have to make an educated guess based on the number of fish you wish to keep together, the size of their tank, and the environment/topography/aquascaping of the aquarium. I have about 700 firemouth right now from one of my 2 pairs. And about 300 Golden Angelfish, from a single breeding pair. My adult angelfish during any spawning period would chew the crap out of even my largest firemouth pair. My angelfish pair are VERY aggressive when they are in breeding mode. I wouldn't risk combining them.
But as a space consideration I have about 150 inch long firemouth young with about 30 of the largest of my angelfish young. (These are about 2" long at the moment.) When the firemouth babies get another half inch to an inch on size, I'll move the angelfish into another tank. For now they share a 65 gallon nursery tank. When they get larger the firemouth school has the potential to fin nip the angels a bit. Hence the move.