Firemouths are one of my favourites; beautiful, relatively peaceful, well-suited to living in a school of their own species, usually topping out at 5 or 6 inches. I've kept them in cichlid communities where they were they smallest species, and they survived and even bred under those conditions, mostly by being discreet and pretty much flying under the radar.
I've had one black belt...back in the day it was called Cichlasoma synspilum...and that was one of the most aggressive and troublesome fish in the tank. It was easily able to maintain itself in a community of notable troublemakers like Buttikoferi, Managuense, Red Devil, etc. It killed numerous tankmates over the years it spent in a 360-gallon tank as one of the main Bad Boys in that group.
Even if the mix you propose worked for a time in a 125, the Vieja will continue to grow much larger when the Firemouths have pretty much stopped. If it doesn't kill them when they are all the same size, which I expect it would...once it's twice their size, I think they are toast.