How do these work together? is it possible to use all of them together?
AFAIK, Real Wildlife adds more things and more things to do with the current wildlife.
SkyTEST is basically a really advanced AI for said wildlife, where they have hunting patterns, don't attack unless you provoke them (for some preditors..), certain spots where they will be more active in Skyrim, has actual herds of deer, etc. I don't think it actually adds any new creatures or spawns (other than stuff like herds of deer, etc).
Automatic Varients, it seems, just dynamically changes textures of animals so there is more variety of what you see (i.e. not all wolves will be black). I think it's compatable with anything, due to the nature of how it gets those varients in there.