The big advantage of the 20 is that it takes a 70-200 still attached to a gripped body. But when it does have that in the bag, there is really only room for a flash and another lens. Yes, you can pack in more but each time you want to get the camera out, it's a PITA. The 20 looks like a big bag, but it's really only big with a larger lens attached.
So, what I'm saying is that perhaps a 10 or 7 may be too small. Maybe just fill the 20 with what you need, and see if it'll fit in a 10. I doubt it'll go in a 7. I always forget how heavy these bags are empty. Stick in a gripped body, a 70-200,flash and a 16-35, and that lot does put some strain on your back!