A pleco might work, because they probably wouldn't be recognized as competition.
Though young haitiensus do a lot of grazing on algae, and as opposed to large meals, do best with a constant supply of food to nibble on throughout the day, so the pleco may more detrimental than not. I liked to have a carpet of algae everywhere in young hait tanks, and it also seemed to hold nitrate levels down.
One of the largest problems most people encounter with haitiensus is the stress induced bacterial infection of bloat.
This could come from cool water, too much food fed at once, high nitrates and/or other tankmates.
I find putting more than 1 hait in a tank (unless its a mated pair) without dividers can be a problem, one will almost certainly intimidate the other into death. And I say this from the experience of losing 19 out of 20 in my first and second attempted groups. My surviving pair spawned many times, and I found the fry would start killing each other at 1", so to raise them up, I'd put 5 young haits in a 50 gal long, and divide it into 5 compartments, one hait to a compartment.

30% every other day water changes, also kept fry mortality at a reasonable level