I don't know that I would call them rare. But yeah you don't see them that often. Care is the same as a regular Gibby. Feed him raw or blanched vegetables like zucchini, cucumber, romaine lettuce, spinach, potatoes, sweet potato's, or if that's too much work, make a good quality algae tab containing sprulina his staple and feed the veggies as a treat. As he gets older you can start to introduce him to meaty foods like raw prawn. Take care to not feed him too much protein when young as it can cause bloat.
"stays small" ha! I love it!
I do fear that he isn't a good match for your discus. Gibbys are one of the few plecs that have been reported to feeding off the slime of slow moving fish like discus. This isn't your biggest concern as it can be avoided as long as he is fed properly. More important. Discus like their water way to warm for a gibby to live happily long term.
Also, the older your Gibby gets the more territorial he will get. In most tanks this is not a problem as long as you don't have other large plecos, but with discus being the timid creatures they are..your gibby chasing them away may spook them into not eating, and as any one who has kept discus for any amount of time will tell you, they will starve themselves to death if not happy.
BNs in general seem to adapt rather well to the warmer water found in a discus tank, not to mention there are plenty of blackwater species of Ancistrus (BN) that live in the same waters that Discus inhabit.