Take a look at the information presented by RodentPro here.

Rat pinkies are 10ish grams which puts them on the same level as a small adult mouse. At this point point there is 30% more calcium in a mouse than a rat, once you get past 50gs (which is a small) in a rat this difference basically disappears...

I don't know all that many breeders but far as I know none of them ever feed mice so this 30% is probably not significant.

If you are still concerned there is a technique known as gut loading which is when you inject supplements into f/t food item that you could look into.