Rubber-Lipped are fine by themselves- they tolerate others given enough territory and hiding spots- I have one in my tank- wish I had a male for her because she occasionally swells with eggs... be interesting to see if I could breed her.
Some of them may have an issue with Plecos of other species- others are fine- from what I've read depends on personality- and probably the exact species because I believe there are several that all go by the same common name.