I agree that a pleco is not the best choice here, especially in a mainly discus tank since I hear they are extremely picky about the water conditions they are kept in. I also agree that most plecos get too big for a 55 gallon tank and it is not just the size they can reach but also how much they need to eat and the wastes they produce. I must have missed something though since I dont see where it says you have two of them already.
I also would agree that there is basically nothing out there that you can buy to get rid of any fish poo, but as far as algea, you might want to look at snails, but be choosy about which ones since they can breed fast and in huge numbers like has already been pointed out and can over run a tank in almost no time at all.
Most discus tanks I have ever seen are usually planted and contain clown loaches. Clown loaches are a good addition, in my opinon, because they will eat nearly anything that is considered food off of the substrate. They also would control the snail population if you had some larger snails. They are also micro-eaters and do not contribute huge amounts of waste all at once. Yes, they do get big, over a foot long even, but they grow so slowly that most people can get away with keeping them in a smaller tank for years and years. I have four in my 55 gallon tank and they havent grown an inch since I got them about 8 months ago. They do like to be in groups of three to five or more so you should try to keep at least three. Otherwise they will be very reclusive and shy. They may also be hard to find for a while after you put them in the tank but they will come out more and more as they get acustomed to your tank.