Guinea pigs are higher in fat than rats or mice. Adult animals also contain more calcium than young animals, making it better to feed an adult mouse vs a baby rat, or a young adult rat vs a baby guinea pig.
That said, people do feed guinea pigs to snakes. It probably isn't harmful just to add variety or help a skinny animal gain some weight.
Here's a link to the nutritional profiles of some common feeders:
https://www.rodentpro.com/informatio...ertebrate-prey