I have the 200c and a 115 cab. I will just say that I love the sound of 15" speakers, especially with the Rumbles. That is why I got the 200c to begin with and why when I wanted an additional cab I got the 115. Sounds incredible! Actually matched the "sound in my head" that people spend a lot of time and money searching for. But I know that a lot of people love the sound of the 10's, and that is great too. In your case, if you like the sound of the 500c with the 10's then going with the 210 cab might be the best choice for you.
As for the size of the cabs, that was a challenge. What I do is lay the 200c on its side on top of the 115, that way the widths match. The 200c and your 500c are the same dimensions, BTW. You can move the feet from the bottom of the combo to the side (they use the corner cap holes so just swap the screws with the feet). In my case, I just ordered another set of feet to put on the side and left the ones on the bottom as they were, just in case I ever wanted to set it upright when I was using it without the 115 cab. I also spun the logo to make it look like it matches better. See attached pic of the rig in my office/practice room. Bottom line, I love the sound and that is more important to me than how the cabs line up. But I am fine with how it looks like this.
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