I've got a 6" L91 Red Beacon pleco with my 5 1/2 " JD male. The last month my JD has grown a lot, when I put them in together, the pleco was 6" and still is. The JD has nearly doubled his size. They were put into my 55g at the same time, and the JD is very aggresive towards the pleco. However, I've made a rock the JD cannot get into for the pleco's safety. Even if the pleco comes out at night, the JD gives him grief. Don't get me wrong, the pleco will fight back and can withstand some damage, but I think I'll eventually end up taking my pleco out.
I took in a 10" common pleco too and tried him in the same tank with the same result. You've already got the JD in the tank, so he's already gonna be very territorial to any addition.
I'd like to hear other people suggestions, as I've not had the best of luck myself. I'm heading towards taking the pleco out and just keeping the male JD in the tank by himself. It's only 55 gallons and I don't want it overstocked when he gets larger.