Having different shocks in the rear may be just fine - or not. Depends on the load you typically carry. I have adjustable shocks, and running empty, the front shocks are set firmer than the rear. On the other hand, towing a heavy trailer, or with a family full of bodies and gear, I set the rear shocks very firm. So if the rear shocks end up being different than the front, you might like them sometimes, and not so much at other times.
As ratmonkey says though, having all four shocks the same may give you the best experience under all load conditions.