You may be surprised how well they seem to pair up. When I brought home my common pleco, he was the same size as my then 8-9" achara catfish. They instantly paired up, and followed one another. Over a year later, the achara has grown to 27" and pleco to 14"....and the achara has eaten many of a fish (learned what he "likes" to snack on), but he doesn't bother the now much smaller~than~him pleco. THey're still 'buds'
Now, I have an adonis and a sydno....catfish. can't think what it's called. They don't tolerate each other and are always having territorial issues (in another tank)
It really depends upon the pair, but should be fine