The mota is going to get big enough to fill out that tank by himself, he is not a community fish so more tankmates is not a good idea, I think you need to do something with your other fish before he gets them.
If you get a satifactory answer will you promise to stop posting this question on other forums in different ways?
There are many canidates that will mix with a motaguense like black belts, butterkoferi, green terrors, red devil, red terrors, urothalmus (also sold as red terror) and I'm sure many others. I would suggest against similar fish like dovii, managuense, fredricstchali(spelling?) etc. That just asking for it.
Now the really impotant part. Your one single Motaguense by it self has the ability to out grow a 55 or at least be big enough to be the only one in it. YOu can house it and others initially when they are smaller but it and it's tank mates will need a bigger aquarium in the not to distant future.
To answer the question you posted concerning this elcewhere, NO, do not put you motaguense in a 10 gallon.