I love plecos and they are supposed to be very hardy, they can handle bad quality water and low oxigenated water, for they can breath atmosferic air by absorbing it with their stomach.
Discus are fragile fish and are very sensitive to nitrates, even more than oscars.I would not advice the combo.
A lot of OFL members will advice against plecos with O's, for the plecos may get the habit of sucking on the slimecoat of slow moving fish.
i must admit i got lots of plecos but NEVER had any of them take an interest in sucking on other fish but if this happened i'm sure my opinion would change.
Just feed your pleco a lot and that will keep them busy.
I feed mine a quarter of zuchinni every other day and they eat until it's completely gone. I feed algae wafers as well, and they still eat all algae in the tank.
Allways make sure thep leco got some driftwood to chew and rasp on, for they need that to properly digest their food.
Give your pleco a good hiding spot or cave so he can hide if he wants to.
A 100 Gl tank shouldt be big enough for 2 smaller or one medium pleco.
This is one of mine, and grows to a foot.
This is a Hypostomus pleco and should be easily available.
allways willing to answer all pleco questions, hope this helps a bit.

PLECOFANATIC!!!