I supinate, and this summer my US fitter took one look at me skating, and moved my blades a few millimeters to the outside AND shimmed them. It made a huge difference (I can hold an outside edge!!!!) Before doing this I asked him about insoles, and he felt blade adjustments would be better. I think most insoles are designed for those who pronate anyway (
). Plus I don't have room in my skates for insoles so......
I do lots of ankle strengthening exercises. I'm sure they're good for me but over the course of a 1/2 season they didn't affect at all my ability to compensate for my supination. Maybe I didn't do them long enough, I dunno.
I suspect this is a common problem though, and the optimal solution varies by skater. When you get your new skates, see what the options are and proceed from there.
If shifting the blade is a potential necessity, then I would make sure whatever boots you get have that option. Leather soles will work, PVC, I'm not sure (someone else here will know though...your fitter should also be able to steer you well on this). As for stiffness, the proshop should be able to help with that, but just in case, SarahSpins gives a pretty good test- with the skates on, stand with both heels on the ground. Bend your knees as far forward as you can. If you can get them over your toes, it's good. If you can't, too stiff. For me, there were some that I was worried about creasing in the store, so I knew they weren't stiff enough. So try the Elle, Freestyle and whatever other models and see how you do. If you have narrow feet, the Riedells may work for you too. It's all about feel at the end of the day.
Take the time you need to do this, and have fun!!!!