Well, it sounds like you already know the answer. Hoplos are pretty big, and a 10 gallon tank is far too small for even a half-grown specimen. I think they can get to be around 20 cm long, and I've certainly seen lots around the 10 to 15 cm mark.
If you have a spare 10 gallon tank why not consider truly miniature species? Gobies, sparkling gouramis, small killifish, etc. There are actually quite a few options. Otherwise, why not use it as a breeding tank for raising fry (that's what I use my 10 gallon tank for). Or maybe even an invertebrate tank with some snails and shrimps?
Cheers,
Neale