Which kind of tool can be used to spread one oil pastel colour over another?
One way to simply keep your thumb clean is to wrap it in a paper towel or cloth. This doesn't really resolve the problem of thumb size, however -- indeed, it makes your thumb larger.

For finer detail work, a tortillion would be better. This is a rolled-up piece of paper that tapers to a point. This is much like Zizouz212's suggestion of an eraser, but perhaps less expensive -- you can purchase tortillions, but it's also possible to simply roll your own from whatever paper you have on hand (here's one tutorial).