Personally I would keep one or the other.
Firemouths or any other thorichthys for that matter can come across as quite aggressive on first impressions. They puff out their gills and display to any intruder on their territory. In reality they are all bluff.They have quite fragile mouths that are very easily dislocated by other cichlids.
If the intruder falls for this,all front assault,a firemouth can keep on chasing and stressing out the other fish until it perishes.
A full grown pair of blue acara in breeding mode could quite easily do some damage to a firemouth if they chose to do so.
I would only normally advocate keeping thorichthys in tanks 4 foot or over.In this instance we are only 6 inches short of that recommendation, therefore it may be worth a try.
Like others have said firemouths are more peaceful in numbers.I would try a group of 6 with some livebearers or have a pair of acaras with some tetras and a few small catfish.
I would definitely not recommend mixing cichlids in a tank with that footprint. It's is a recipe for disaster.