Yeah, it'd work, providing that your tank is big enough. I'd say at least 30 gallons, preferably a bit bigger. Provide logs and plants. They should be fine. However, I'd probably put in the port acara first or at the same time as the firemouth, and try and get them small.
Firemouths are not as aggressive as some people think. They are mild tempered, and could live with Acaras, like VoodooChild said, providing you have a big enough tank.
Firemouths in captivity only reach about 4 1/2 to 5 inches. I have kept many amongst the years. Acaras are pretty much the same. 30 gallons is more than enough, if provided rocks, and hiding places for the fish.
lol 10 inchs eh. no I'd say the same along the lines of 4-6" for both the meeki and acara. 30 gallons would be plenty for these two. Aggressive nature isn't a big part of their existance so long as its one of each and not pairs. Even then though I don't know if that would be too much trouble, but I wouldn't risk it.
You might want to stick with those 2, or maybe something else mild like a festivum. What aboul some pictus cats or one of the tougher L numbers like an L-14?