There are really only two ways of cross-processing -- E6 (slide) films developed in C-41 (print) chemistry, and C41 film in E-6 chemistry. I do not call developing any color films in b&w developers xpro, for the simple reason is that you are only getting a b&w silver image, not a transference of different colors and saturation.
In my opinion, the best results are from E6 films in C-41. With E-6 minilabs disappearing from the scene today, it is less likely that anyone is going to go the C-41 into E6 route. In fact, because C-41 is still the only available local process for many people, I would expect that xpro might become more commonplace for people with E6 film to shoot, if the lab will allow it. (I read somewhere that so long as the total rolls of E6 through a C41 minilab do not exceed 20% of the capacity, the chemistry will be fine.)
When should one use xpro for E6? If your slide film is expired, it's a great time to use it for xpro imagery. If you have Tungsten-balanced film such as Ektachrome 64T, try it with night shots and xpro. Old Fuji Provia 400 looks contrasty and grainy, and has a very "vintage" 1970s look. There are plenty of odd E6 films to try out, and if your lab will process them in C41, you can have a lot of fun seeing what films give the results that please you the most. Locally, I thank Huron Camera in Dexter, Michigan for doing the cross-processing.
Subject-wise I recommend sunny, high-contrast scenes which seem to offer the most colorful and contrasty images. Subdued scenes can look muddy and exhibit less of an obvious xpro effect. In any case, play around and have fun.