On the shrimp side, it would have to be Amanos...they are very tough cookies if you have "adventurous" fish already. One single Amano of mine, not even the largest one, threw out 3 Bandit Corys from their hidey hole and took it for himself. They're avid algae eaters and very interesting creatures and don't add much to your bio-load either.
I just don't think I'd recommend any sort of plec for that size tank.
6 Sterbai Corydoras might be a good addition, too. Fun to watch and they're not quite as skittish as some of the other types of cory.