In my experience, Leporacanthicus are not aggressive (I have five different kinds now:
(L91) (these are a good 20cm TL now),
(around 10-12cm TL),
(10cm TL),
(at least 20cm TL) and
(around 12-15cm)). However, they are somewhat territorial, so if you have a group of them, they will be in separate parts of the tank - or at least, they will keep a little bit a of a distance. I've kept mine with a variety of other plecos, and not really had any issues - as long as there are enough hiding spaces, I haven't had any problem with them being overly harsh with their tank-mates.
I have never kept discus, but I have kept Leporacanthicus with Eartheaters, which is another kind of "fairly gentle cichlid". I've had no issues with these.
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Mats