The Mota and Pleco are the two largest fish, both around 12-14".That's a really hard question to answer man lol If you got a male and a female and they paired off that could be a very real problem. Hoplarchus get really big too-bigger than your Mota, their could be a problematic change in hierarchy. I believe part of the reason you've had so much success with your Mota is because it's easily the biggest fish in your tank-or am I off?
I also wouldn't just buy fish to justify shipping costs when it could lead to problems down the line-just something to think about.
The reason I've had so much success with my Mota is because he's a wimp. When the Pleco tries to start fights with him, he swims to the corner, and he's still a bit iffy about me walking up to the tank. If I get about 6 inches from the tank he darts off.