Anything in the Amph family is probably going to be too risky if you're hoping to keep the SD's and Plecos. As for a large single fish, you could probably do something in the Vieja family (Argentea, Synspilum, Zonatus, Bifasciatus, etc). They're large colorful Cichlids that are usually less aggressive and leave smaller tankmates alone. There are simply too many options/possibilities to include in this thread, but you should get some great ideas from members on this site.