Hi welcome to the forum
So first thing with an American Cichlid tank - it is best to keep South Americans or Central Americans, it is possible to mix them but generally they have different water requirements and their behaviour and aggression is different and displayed differently which can make things harder to manage.
From your list the Firemouth and Convict are from Central America and most Acaras are from South America for example - there is the Green Acara - Aequidens Coeruleopunctatus that comes from Panama but otherwise Blue Acaras, Royals, Yellows etc all come from South America.
In a tank this size I think mixing 2 individuals of the species you've mentioned so far won't end well... Convicts are the most aggressive but either way you are looking at 2-3 mid to upper level aggressive cichlids and I think there are better choices if you hunt out a few specifics.
I'm asumming you have neutral water, but if you have soft or specifically hard water there may be better or other choices. I'd go for Central Americans and I'd look at the calmer Amatitlania species like Cutteri or Nanoluteus, maybe Sajica. I think Rainbow Cichlids are worth mentioning here as well. The tricky thing is working out the game of chess that cichlid stocking is... if you do 2 males of different species you will get a dominant and a sub and the sub will be very miserable, if you get a male and a female of 2 species they will breed and getting a 3rd species is quite hard in here because you would have the most success with another species that does not look like the first 2. The best I can think of suggesting is a Neet but hard to sex as they are monomorphic where as the others are quite easy.
What are your thoughts on doing a group of something like Nanoluteus or Cutteri - Honduran Red Points are worth a discussion in here too or a pair of Rainbow Cichlids?
I think if it were my tank I'd be considering a small group of Nanoluteus maybe 1 male, 3 females - you might manage 2 males though as they are not too aggressive and then pair them with a nice livebearer like some Swordtails or maybe some kind of Goodied - Skiffia, Tequila Splitfins etc.
Wills