I only took a couple frames, so they've each started with two nearly full supers of honey, but they were active well into December this year, and as this is my first year, I can't guess how much they've eaten.

What temp is the safest minimum for opening hives? Should I just wait till they're active?

By supers do you mean Honey Supers (medium or shallow)
or do you mean Hive Bodies (deeps)

In Norteastern Ohio, bees need 80lbs of honey to make it through the winter...That is, 1 deep full of honey!
This deep should be above the clustering bees inside a 2nd deep. If you have 2 honey supers above the 2 deeps, the bees SHOULD be fine.

Next time it is warm, 40 degrees or more, look inside the hive and figure out where the honey is, as compared to the bees. If there is no honey adjacent to the bees, or directly above them....Put some frames of honey adjacent to the cluster. Bees will not abandon brood (which they should have started raising by now) to get honey. Even if it is only a frame away. More bees die of starvation with plenty of honey near by than most beekeepers care to admit.

These well-established hives were on the property when I bought it, and as it happened, the old fellow who kept them was looking to sell out because of health problems, so I could hardly resist despite my scant knowledge.

I didn't realize how limited the bees' movement inside the hive is, now I'm more worried than ever...

There are three of the smaller brood boxes(medium super size) and two shallow honey supers per hive, and in November they were all pretty much full.

I think this spring I will add a regular hive body to each, 2 if i can pick them up at a decent price. I'd only ever want maybe one excluded super honeywise for myself, so yes, now that i've been thinking it through, the deep boxes make more sense for how I'll be keeping the bees.

THe guy I bought them from used the smaller brood boxes because of his health problems making it hard for him to handle the big ones, but he was a very good beekeeper. It's good that I can easily move full frames into those boxes, since they're nearly the same size as the supers, but he always had to feed them pretty extensively.

Thanks so much for the help, matiwatu, I'll go ahead and buy feeders along with the hive bodies, cause i doubt those three brood boxes add up to two deep supers. I'm a nurse, middle-aged but still pretty strong, ;~) so i will have no problem moving the big boxes.

I do have two large empty boxes that he used for feeders, maybe I could fill those up with the frames from the supers to give them a new place to cluster?

Come on Spring!!!