Kawi used a rear rack and side rack from two different manufacturers to make them ostensibly incompatible, to prevent their concurrent use, to prevent possible overloading, and therefore legal issues.
To answer your question, yes,they can be used together but, you must cut off and file smooth the integral metal grab handles that come as part of the Kawasaki (Givi made) side racks.
I happenned to prefer the design of the Kawasaki side racks over the SW-Motechs (and I got a good deal), so I have the Kawi side racks mounted with the Kawi rear rack (SW-Motech, it came with the bike).
E21's won't work with the Kawi side racks. The Kawi side racks are "PLX" side racks which mean they only work with the V35, undercut, side specific bags...this doesn't mean you can't modify things if you have the time and ingenuity...
The SW-Motech side-racks would probably give you the most flexibility. They have a quick disconnect option - Kawi racks are definitely not quick disconnect, but they also can't be removed by a thief, without a torch or hacksaw, anyways.