wife asked me about putting in a flat rock in bottom of cage, the gliders would walk or run on this and it might help keep their nails trimmed a bit.
Is this ok or a bad idea?

I've recently been considering that very same thing! It worked with my lizards, both my Leo and my little Rankin's Bearded Dragon.

I've tried wood in the glider's cage, because I read somewhere that when gliders climb and jump/land tree to tree, it helps keep their nails under control. Of course I realize in the wild, they need somewhat longer nails anyway, but they'd probably would be way out of hand if it weren't for the trees. This hasn't really helped in my trio's case, maybe it's not enough wood or enough activity with wood, or maybe they're just too long for captive gliders

But, I have been contemplating trying to find some flat rock, and placing it on the floor of cage, at floor level feeding station. I figured this would be the most likely place that they'd spend time on it. They do go to the floor of their cage, other than to eat, I keep it interesting with their floor toys they love (tree house, Noah's arc, wheel, and now treadmill, sometimes a toy bin as well) not all at once, I alternate them in the cage. Even so, I just don't see them hanging out on a rock unless they were eating.

Please keep me posted if you put one in there, and how it works. I'll do the same if I get one soon, but I don't think it will be that soon.

That sounds nice Karen, I've always thought I'd like a large branch in my glider's cage, but haven't managed to find a safe one. I did try a big crepe myrtle branch once, but it wasn't quite big enough and I had a hard time securing it in their cage. The few times I tried, they didn't make much use of it. So out it went.

I too have considered a rock, but one that was vertical. My biggest concern with is securing to the side in a way that would be easy to remove for cleaning.

Redbud branches are safe too. I have one that when I prune it, I will put in the cage for a few weeks/ months, depending on how smelly it gets... If I clean it diligently, it lasts longer.