Give it a go with the two different sizes. You never know what will or will not work out until you give it a go.
I have two different size subs in my setup, and was lucky that they worked out. Not ideal, and not easy to integrate, but with some luck, not impossible.
I had a problem with a null around 35hz with my one sub setup that I could not get rid of. In a 'what the heck' moment, I hooked up my old sub, put it back in its old position, and two weeks of tweaking later, I greatly reduced the null.
