Get a trio of petricola (or the cheaper nigriventris), or more if you can. Even with sociable species, getting two of a fish can result in the more dominant animal "bullying"/outcompeting the other. Larger numbers will allow social interactions to diffuse.
Another alternative is a single eupterus, which is also one of the cheapest species. While they're social animals in large enough tanks, they also enjoy having some territory to themselves and a single fish will not be stressed.
You have plenty of space, 85g will support all but the very largest synos (S. clarias, S. batensoda etc.) As long as these are the only fish in the tank, you'll probably have an easy time since low bioloads in large tanks is a proven formula for success.