I have done exactly what you are doing, plus also have removed studio photos from the 1920's, 30's & 40's from those old pasteboard folding stand-up frames that the pro's used to use. I used the Creative Memories multi-tool when there was a fairly free corner and very gently worked my way around a bit at a time. Most came off very easily with only that. When there was a section that was really stubborn, I tried the dental floss--also working slowly and gently until I knew for sure it would come loose without damage. I also have some stiff plastic spatulas that came with my Cuisinart many years ago which work very much like the CM multi-tool only with a broader span to work with--sometimes works well and sometimes does not depending on the individual photo.
I have used Un-Du on my scrapbook pages and when removing rubber stamps from wooden blocks. I was reluctant to use it on the old photos as some of them were already starting to silver or deteriorate from a reaction with their mount so I just didn't want to add another chemical to the mix not knowing how well the original processor had "fixed" the image. It would be well worth a try though to experiment on one photo that you aren't deeply attached to, although as pointed out above it would be best to just scan everything first if you have that capability available.
Whatever you do, be patient and don't rush the process, whichever you use.