I have a Gitzo 3530S which is definitely plenty for the 500 and beyond. I also have a Kirk BH1 which I used to use with a Sidekick (now I have the full Wimberley head). The Sidekick works fine with the 500 so you'd be good there. However, since you also mention 600mm, just be aware that the Sidekick is not rated for that.
Thanks Liz, it's encouraging to know the Sidekick + BH1 get your thumbs up.
I am not up to humping the 600mm around, but the longest I have at present is the 300 with 2X (or stacked 1.4X), although relatively light I don't feel my old Benbo is well damped enough for that focal length. Apart from that handling an unlocked Benbo is like trying to manage a drunk set of bagpipes! 





