1. 'we are having conversion issues from the single rail to regular unevens.'
Welcome to giants, front and back!

2. 'Should we have not used the single rail so much?'
I'm not sure you could ever use it too much if you still have girls peeling under any conditions.
3. 'Should we just get them up there and slow them down more from the handstand so they don't peel?'
Absolutely. They should never peel.
4. 'Should we just keep drilling and come back to this?'
If the drills look good, do both. If not, back to lots of drills.
5. 'How do we address the low rail?'
After they look great on the single bar, I introduce the idea of the low bar. I use two coffee cans filled with cement and a PVC pipe upright. I put them behind the high bar with under-wrap across the two, to act like a low bar. I put them so far back the girls couldn't hit it if they were fully laid out. When they get used to that, I move it ever so closer. When it finally gets in their range, they feel pretty good about it, knowing it wouldn't hurt. As they are learning, they will break it many times, but with no pain or fear. In fact, it's kind of fun for them. (please don't use that unbreakable yellow underwrap! I won't digress, but wow that was a thrill ride!!)

When that's good, I do the same with the low bar. I move it out of their range and cover it with a mat or Tkachev pad and then slowly work it in. They will probably tag it a few times while learning, but once again, no pain and no fear.
As for front giants I could go on forever (but luckily for you I won't)

Just a few tips. They need tremendous grip strength, little girls and little hands can do them, but it's far more dangerous, I believe in having their thumbs around the bar the whole time, spot, spot, and spot. When they finally don't need a spot.......SPOT!