I like the rrs gimbal but isn't it like $800? Lol I'm looking for something that can maybe clamp on or something so I don't have to change heads if the tripod which isn't a big deal but of course a full gimbal would be much more sturdy
It's not cheap, true. But if you have a need to do nodal panos, it's cheaper than a full gimbal and a nodal setup. Else, save $200 and get the Wimberley II. For a 500mm lens, I'd go with a full gimbal over a side mount.
The Sidekick is fine for the weight of the 500mm. The most important part of this setup is a robust ballhead. From my experience, the ballhead's weight capacity should be at least 39 lbs and more. If you have the budget, the Wimberley II head is the best solution. The Wimberley II's weight capacity is 150lbs.
It's not just about load capacity. You also have to mount the lens into the clamp and balance it - with a full gimbal the cradle supports the lens while you do that, but with a side mount, you have support the lens with one hand while you mount it, and that can be hard with a longer lens.
The problem with the full gimbal is taking it off and putting it back on the tripod when you need just a ballhead. The on and off places a strain on the tripod mount screw.
It's an added cost (of course), but RRS offers a leveling base with an integrated clamp, and dovetail plates for heads (the 40mm one fits the Wimberley II) - that allows fast switching between ballhead and gimbal. Or, with the RRS full gimbal, you can slide the FAS clamp onto the vertical rail (same way you'd set up a pano) and mount the camera's L-bracket to the gimbal.