I think your dimensions are wrong. If the tank is 12in long then no it wont work. If it is 30 long than you will have a lot better chance. I have never kept Blue Acaras and firemouths together but they have similar temperment so it should work out.
I have three blue acaras and one firemouth in a 55. They're fine together. Sometimes the firemouth shows some aggression but they never come to blows. Blue acaras are peaceful, curious, but skiddish fish and work well with fish that aren't smaller than them.