I've kept Rams and Cory cats together before, in a 29 actually, and there were indeed territory disputes. Nothing major, just the male Ram shooing the cories away from the eggs. Never any kind of injury though.
I'm not looking for a plec that will actually clean anything really. If it does, great, but I want a nice looking plec for the sake of having the nice looking plec. They're interesting and different. I've been reading more on Gold Nugget plecs and some do stay in the 6" range, which is perfect for what I want.
For my water, I'll be using an RO filter. For saltwater, that's a great thing.. But for fresh, is it still? Is the RO going to strip the trace elements out of the water and starve the plants? I'm sure a good fertilizer will fix that, but I don't want to build in a problem if I can help it.