Since nobody has answered, I will.
Yes you can use two different sizes/resolutions for extended desktop. The only time this won't work is with span mode, where the entire desktop is rendered across two monitors as if it were one (mostly used for gaming)
I have a 15.6 inch 768x1360 monitor and its fine with my 1920x1200's. Only problem is that windows tend to shrink/expand when they cross monitors unless they are perfectly matched.