I'm running a pair of Turnigy 3S 1600mah 25C lipos in parallel in my Mini-E-Revo. Works really well and provides a ton of run time.

If you have a high amp draw setup, running different C ratings can cause the lower C rated lipo to overheat. If your amp draw is not that high, just watch the lower C rated lipo and make sure its not getting warmer than the other. The safest thing to do is use the same C ratings.