Unless you can quarantine, there will always be a risk.
If you can't quarantine, but still want to risk it, then study the fish you are buying for typical symptoms.
Hanging at the surface,fast gill rate,clamped fins,white spots,clear/white covering of the skin,little white growths on fins or mouth,stringy white poo,flashing off rocks or substrate, etc.
All of the above are typical poor health indicators. A fish showing any of these symptoms should not be bought.
Hoplo's are a tough hardy species and normally quite resilient. If you find some and they look active and healthy and feed well,I would take the plunge and monitor closely for any unwanted symptoms.
I'll throw my extra sponge filter in a tank and watch them for a few days, just in case.
