I agree with Bob that calcium will be diluted while free ranging unless extra calcium, like oyster shell, is supplied.I wanted to take a moment and think this through before I responded. I am going to ask for one allowance. I am writing this to address the difference in shells between the hens. If I am missing the mark and you are trying to address a soft-shelled egg, please forgive me and let me know.
The Roadrunners are Legbars and are only 1 year old. They are in the peak of their laying cycle and should remain there for a couple of years as they are not over producers like Leghorns are.
Your other hens are Production Reds, are they not? I know Maggie is an RIR if I remember correctly. Production Reds are just like the Leghorn in that they produce a lot of eggs early in their lifespan. The Production Reds are the same age as Maggie right? You already know that as it relates to egg production, Maggie is elderly. I submit that your other hens are as well. Egg shell problems are common as chickens age (that's why soft-shelled eggs increase in frequency as a chicken ages). I would suggest that the thinner shells are likely a by-product of this aging.
Now if I am wrong and they are not Productions Reds let me know. They are not old by chicken age standards and perhaps we should be looking at this differently.
I do think that supplementing their calcium intake is not a bad thing. Do you offer oyster shell as well? (I'm sorry but with so many soft-shelled issues running around I can't keep the details straight in my head).
Keep this in mind. The layer feed offers everything they need nutritionally for laying eggs provided that is all they eat. Anything they eat outside that will unbalance the diet. I am certain that they seek out what they need. Shadrach has told those stories many times and I have trust in the ladies to do that. But if high calcium sources are not available they will actually lower their calcium intake by eating non-feed sources of food. That is why even when my ladies were on layer feed, I had oyster shell available for them. They did not use much but mine have always eaten some oyster shell.
I hope this helps. Please correct me if any of my assumptions were wrong.