Before you buy plates you need to decide on what type of clamp you want to use on the Q20. Markins heads come with a knob clamp and a lever clamp. The clamp is the part of the ball head that you will attach the camera or lens to.
If you plan on going with a lever clamp it is recommended that you stick with one manufacturer when it comes to the camera plates, lens plates, lens replacement foot or L-Brackets. This will eliminate the potential of having to adjust the tension on the lever due to differences in manufacturing tolerances in plates made by different companies. The last thing you want to do in the field is have to continually adjust the tension on your lever.
With a knob clamp the tolerance issue does not come into play since you manually tighten the clamp. This gives you flexibility in brand/cost choice and at times can impact availability of a specialty plate.
I would suggest a L-Bracket for your camera, this will allow you to go from landscape to portrait keeping the camera centered on the tripod. You will also need lens plates or replacement foot (if available) for your 70-200 and 50-500.
You have a Sigma 50-500, based on that lens I would recommend you consider a Series 3 (32mm) or 4 (37mm) tripod. A series 2 (28mm) may be adequate, but just, for the 50-500. Feisol makes an excellent Series 4 (3372, 3472 systematic type) that weighs about the same as the Gitzo 3 series systematic and offers the same performance for much less. I base the above statement on my personal experience testing a Feisol 3472 against my Gitzo 3541LS using a gimbal head, D300 and 500/4 AF-S lens. The Feisol is slightly bulkier since the upper legs and top plate are larger but that may not be a negative since the new Gitzo series 3 comes with a larger triangular top plate vs the round top plate of the previous 3541LS. So bulk may be about equal between the Feisol and new Gitzo systematic version. Feisol has a large following and their customer service is great.
You have chosen a great head, that is half the battle. Good luck on the legs.