Most of the time, Plecos and Polys aren't a good combination. I've only seen Senegals and Plecos work, but I think it's because Senegals are the most active.
Ornates can be more temperamental then others, but like mentioned, it really depends. The last Poly I bought was an Ornate. I placed it in a tank that was already established with other Polys. I think because it's the newbie in the tank, it's not aggressive towards the other. I had another Ornate before that I kept in it's own tank and when I tried introducing another Poly, it didn't want anything in that tank.