My thoughts are about 11 to 11.5 for Sweetums, and if Thog were to happen the same height. Thog's sideways mass would more than make up for the height differential.
I dunno, I would think there would be at least a slight height difference, even if it's only an inch.
11-11.5"? Man, first you had me going about the 9-9.5" one, thinking how great that would look, then I was stoked on the 10 inch Sweetums, now it's 11-11.5 inches! Man, I don't think I can take any more! Gotta re-visualize everything all over again!
Seriously though, I would make Thog just a little bit taller than Sweetums. By the actual scale I calculated Thog would be approx. 2 inches taller than Sweetums, 1 1/2 inches would be acceptable, one inch would just be enough. Also, I've pictured a 9, 9.5, and 10 inch Sweetums with the other figs; 10" will do, trust me. At first I didn't think so, but then I was only visualizing him being taller than the other figs instead of taller AND more massive.
Have you ever seen the book "Two for the Show"? The offer still stands if you want to borrow it. What it is is illustrations of Sweetums in scale to the other Muppets that might be handy.
Just my two cents once again.