I would try and get one the same size as your endlicheri. I've kept Bichirs with a Pleco of the same size for a while now and they get along perfectly fine.
As mentioned before, the Pleco may try and suck on the Bichir's slime coat and it has happened in my experience but it's mostly a curious or accidently thing when feeding. I've never seen ANY aggression between the two fish and I doubt you'll have any problems either, just again try to match them in size.