Unfortunately there is not currently a way to randomise animations using only RailClone. However, I notice you have posted in the Forest Pack forum, so I would recommend using Forest Pack for the crowd and RailClone for the seating and other stadium elements.
Forest pack will enable you randomly select from a series of time offsets, you can find some information about this in the documentation
and the Windy Grass tutorial
If you would like to ensure that the crowd created with Forest Pack aligns to the seating created in RailClone then you can create triangular markers with a RailClone object that are then used to place and align characters using Forest Pack. This post
describes how to do this and includes a scene file you can download and check out.
I hope that helps, please let me know if you have further questions about this technique or have other questions about using Forest Pack and RailClone together.