I put some used Superfeet insoles in my new Danners and they slipped just a little on the downhill part of a hike (1/4in to 1/2in) but they're very prominent insoles. Is this a huge problem as far as in-shoe wear goes, or most importantly, preventing blisters? I don't really want to glue my boots or modify them for insoles, I'll just go get some new ones if needed!
Most insoles are either sized to fit, or cut to fit in your shoes, if your insoles don't fit right then you should probably buy new ones. But first I would question whether or not your new boots are the proper fit. If your insoles are slipping, chances are that you have more room in your shoes than you need, and your foot might slide around with or without properly sized insoles, though getting the right insoles can also improve the fit of your boot.
Insoles will only give you blisters if they're sliding against your foot, or causing your foot to rub against the sides or uppers of your boots, if they shift a bit in your boots when switching from uphill to down hill, it's not a big problem, you'll maybe put additional wear on the inside of your boots over time. You could try a strip of double sided tape or gluing them in and see if that helps, or just tighten up your laces a bit more to keep your heel in it's cup on the down hill.