I've had Discus for over 18 months now and we also keep them with a plec. The plec never has anything to do with the discus and lives under the bog wood, the only time we ever see it is when it comes out to eat bits of the food that the discus leave. It never bothers them
We have eight in a 65g tank
1 Blue saph
2 red turks
2 blue turks
2 pigeon blood
1 cobolt
1 plec
2 filters, int and ext
Either a 2.5g water change daily or a 5g every other depending on work.
RO water only as our tap water is 8.4ph! and I would never trust it anyway.
They are well worth keeping, ours go nuts when you walk into the room, demanding feeding or just a stroke, they are so friendly, I tried the other week to see if I could pick one up by hand, I was shocked when the blue saph let me, calm as you like just above the water for about 2 seconds, I slid it straight back in and it went straight back to 'feed me' dance they do
Can be a bit of a pain at times, like when you want a holiday, you need to find someone very reliable to do the water changes and we tend not to feed them on the one or two days we manage to get away a year(as we also have another tank and various other animals)
Hope this helps, if need more info let me know