Regal Rat is a really good base mix, but mice like a lot of variety in there diet. As omnivores they can eat just about anything! This is my mix, it's homemade, with just about everything available at the supermarket. A 'scoop' can be any size container, depending on the quantity you need to make up. My 'scoop' size is probably 4-5 cups.
Mouse/Rat food - 3 scoops
Premium Bird Seed - 3 scoops (more seeds, less grains + more variety)
Eagle Pack Dry Food - 1/2 Scoop
Buckwheat Pasta Spirals (1/2 scoop) (Tri-Colour pasta spirals are also good!)
Shredded Wheat
Puffed Rice
Wholegrain Ryvitas
Rolled Oats
Plain Corn Kernals
Weeties
Green Lentils
(3 scoops combined)
For treats they get:
Cooked chicken
cooked pasta
steamed veggies (carrot, corn, peas, beans + broccoli)
raw veggies
Dog Biscuits
Plain Yoghurt
They can also get mealworms as a treat.
All About Mice Feeding - Staple Diet
This diet is based of the 'Shunamite' diet, and my little meecers love it.
PS Welcome to the wonderful world of mice ownership, they really are a joy to have