I keep cories with my discus. I'd've thought discus would be likely to eat smaller shrimp, but that cories wouldn't. Neither would be likely to eat snails IMO. Soft acid water tends to mean that snails and shrimps suffer from poor shell /exoskeleton developmnent; a pH of 6.6 should be OK but maybe not ideal for this reason.
Re the farowella, I've never kept them but have heard they are a good tankmate for discus. Some people report plecs removing the mucus from discus, causing them stress and leaving them more open to infection. Not everyone seems to expereince this; personally unless your desperate to try it and can rehome the plecs if they give a problem, I'd be inclined to steer well clear of them.
The only type of other cichlid I've kept with mine were Blue Rams, which were fine. I really can't answer for other types though.
Re the farowella, I've never kept them but have heard they are a good tankmate for discus. Some people report plecs removing the mucus from discus, causing them stress and leaving them more open to infection. Not everyone seems to expereince this; personally unless your desperate to try it and can rehome the plecs if they give a problem, I'd be inclined to steer well clear of them.
The only type of other cichlid I've kept with mine were Blue Rams, which were fine. I really can't answer for other types though.
Are U saying that my snails may die off on their own because my PH is 6.6 and soft water?That would be great; I dont really care if I can keep shrimp,just don't want the snails to take over.Don't really want loaches either because they dont naturally occur in discus waters and I want to do the biotype thing.
so far snails and cyclops are all I got!Where do you find a snail trap?Never heard of them.Thanx 4 your help 
