Can I carry out a probabilistic analysis on a single slip surface that I define?

Yes, this can be done in Slide. What you need to do is turn off the automatic creation of surfaces. If you are doing a circular analysis, simply do a Grid Search but do not define a grid. Then, simply define the single circular surface of interest. If you are doing a noncircular analysis, set the search method to Path Search and the number of generated surfaces to zero. Then add your single surface.