I run Dempseys and Firemouths in the same 55, two of each, with a Pleco. No problems so far, though I am pouring a little slab under the floor so I can reinforce it with floor jacks and move all the fish to a 125.
Okay...filtration on a 125 containing six Dempseys, four Firemouths and a Panaque nigrolineatus? Two big Eheim canisters, two Tetra Whisper 3060s, probably a diatom canister to run once a week is what I'm considering.