I'm a big pleco fan and I have 4 different kinds. Never once have I ever seen them try and latch on to other fish. These are scavengers not hunters. Now a CAE yes they will chase fish are latch on. if your Carnivorous pleco is starving it would go for the fish. But for the most part plecos keep to them selves if all their needs are filled.
Some mature larger plecos can become territorial and might chase fish away from it's cave or food. All fish have personality I guess you could get a crazy one.
I don't see why you could not keep a small pleco with Discus like a clown pleco , Bn, Flash, or pitbull pleco. If you have a really big tank I think you could keep a large pleco with Discus as long as you make sure too feed your plecos the proper food it needs. Also you need to give the Pleco a cave to call it's own they need a place to feel safe.