Be careful with Oto cats together with Discus. Often they're ok, but occasionally one or more will take a liking to Discus' slime coats as part of their diet. I've had that experience - twice - no more Otos in my discus tank!
The ottos and discus slime coat is a valid warning but in my experience, its not that big a problem, the discus shake the ottos off pretty easily and at least in my case, its never stressed the discus out.
You could try bristlenoses with Discus. I have a few in my discus tanks with no problems whatsoever
Agree with josephl. BN's are good with discus. Also, SAE's (siamese algae eaters) - take care not to get CAE's, (chinese algae eaters), which I understand can grow quite large and be aggressive. Flying Foxes should be okay - they're very similar to SAE's, but the dark lateral line ends before the tail, whereas in SAE's, the same line extends into the tail.
Island Pets Unlimited has some SAE's at the Burnaby store for like $2.99 each, got a couple a couple weeks ago and thee guys are eating machines. I recommend getting 2 as they seem to always be together, maybe its just the 2 I got lol either way good fish for the job