I've had (and still have) various Manfrotto tripods and heads. None of them are compatible with the Wimberly/Arca Swiss style QR clamps and plates. It's a dilemma I've had to deal with recently. I too have the Canon 500mm, and use it with the Manfrotto 393 gimbel head, which comes with their own heavy duty QR clamp/plate system
(357 Pro Quick Release Adapter with 357PL plate) which is not as small as the Arca Swiss type clamps and plates. However, in the past week I finally bought an Acratech Ultimate Ballhead which comes with their Arca Swiss style clamp, so I had to finally break down and spend the money for two plates for my camera bodies. I settled on the Wimberly P-5 plate which is non-specific to any particular camera body - it's a fits all style - because it had the safety pins at either end so the plate wouldn't slide out accidently. You have to open the clamp all the way to take it out. There is a point to all this and I'm getting there. (Here is a link to the Wimberly site - at the bottom of the page is a link to their VIDEO explaining how the safety pin feature works
http://www.tripodhead.com/products/camera-body-main.cfm )
After using the Manfrotto system (which was originally designed for heavy 3-tube video cameras back when digital hadn't made it yet) I would not put anything less on my 500mm lens. The wedge along the side of the Wimberly P-5 plate is rounded smooth instead of coming to a sharpish edge, and the Acratech clamp has a sharp V notch to it. Thus, the plate and clamp don't overlap much when clamped. The lack of clamping surface between the two doesn't give me much confidence in using that style system with heavy lenses. The Manfrotto 357 however has a deeper wedge with more surface, and it is foolproof. You can ONLY slide the plate in. The clamp does not open. It has a spring loaded pin that pops into place as you slide in the plate and the plate will not come out unless you press the release pin. It is a foolproof safety feature unlike what I've seen with the Arca Swiss/Wimberly style stuff. You might forget to tighten the clamp lock, but even if you forget, the plate (and lens) will slide a little but it WILL NOT come out accidently.
Don't get me wrong, as I'm not knocking the Arca Swiss system. The point I'm making is - be sure whatever plate you get to use with your clamp provides a good fit and overlap between the plate edge and the clamp jaws. And if you can get something with safety pin stops on it so the plate cannot slide out accidently, it'll be worth whatever you pay extra for it for safety's sake.
If anyone else has experience with this issue, I'd be very interested myself in further info on better combinations of Arca Swiss style clamps and plate combinations than the one I ended up with.