With Parachromis species, as they get older, they like to have a certain territory size and keep any fish out of that territory, even non-cichlids. I've had a 6" jag prevent any fish get within his territory which was 4 feet long in an 8 foot long tank. The silver dollars that I had were miserable in the tank with the reduced swimming space. With motaguense generally being more aggressive than managuense, it's not far fetched that you may end up with only the motaguense in the 6 foot long 135g tank within 3 years.