I have 4 600EX-RTs and 3 Einstein E640s. I love the Canon wireless system, and wish that Buff sold a compatible receiver for the E640s.
Methods for mixing Einsteins and 600EX-RTs:
All PocketWizard: A FlexTT5 and AC3 zone controller on the camera, a FlexTT5 for each 600EX-RT, and an MC2 for each Einstein. Expensive, but gives you complete control over output of all lights, in 3 zones, from the camera. As much as I love the Canon radio system, there is nothing easier than the physical dials on the AC3 zone controller.
CyberCommander + Canon: A CyberCommander for the camera, connected by a short (included) PC cord. CyberSync transcevers ($30 each!) for the Einsteins. Then put either a 600EX-RT or the ST-E3-RT on the hot shoe, and use that to control the Canon lights. This complicates things, as you now have two radio controllers, and you have to dial in the Einsteins on one and the Canons on the other.
CyberCommander + PocketWizard:: A nonsensical option; use the CyberCommander (via PC cord) to control the Einsteins, and PocketWizards (via FlexTT5/AC3 in hot shoe) to control the 600EX-RTs. You wouldn't do that, though, sine the Canon lights have their own radios built in.
Canon + optical slave: A 600EX-RT or ST-E3-RT on the camera controlling the Canon lights. The Einsteins are set to optical slave mode. You don't have control over the output of the Einsteins from the camera, and you are at the mercy of optical triggering, which can be a tad frustrating.
CyberCommander + optical slave: A CyberCommander in the hot shoe, controlling the Einstein lights. The Canon lights are set ot optical slave mode. You don't have control over the output of the Canon lights form the camera, and you are at the mercy of optical triggering, which can be a tad frustrating.
I usually go All PocketWizard. It hurts to bypass the Canon wireless system that I so love, but the mechanical dials on the PocketWizard AC3 Zone Controller are so simple and quick that you don't really have to think much. It's much quicker than the Canon menu system.