I have a clown in with my 15" silver arow, a 12" indo tiger, 12" royal knife, 8" Oscar, 12" tsn and other fish. As long as you have a lot of cover for your clown, you should be fine.
I have a huge piece of driftwood in the middle and rock caves all around, he usually is hiding i just heard if they try to eat it the spines will kill them
Plecos are a lot of times used with cichlid because of their armored appearance. People think that they will be protected but that is false. If a cichlid feels the need to dispatch a pleco it can and mist likely will (granted the cichlud is large enough to do so) A lot of people have no problems with them. I had a 4 inch jack Dempsey (I still have her) that killed a 6 inch pleco. I went without one for about a year and just bought a new pleco. It is slightly shorter than ny Dempsey pair and its still alive after a month. They bully him a little bit only If it goes to their side of the tank. Hopefully he sticks around because he's prettier than my last one!