Mix is a bad idea .... they pick and choose so do not get good nutrition, there are unhealthy things in mixes, there are things in mixes that rats can not digest, and sometimes mixes contain things like dried corn which can contain mold
(plus corn is very unsafe unless organic/nonGMO)
A good quality block like oxbow regal rat available 24/7
daily vegs
and healthy treats like a piece of fruit, berries, melon, a piece of organic cereal, cooked grains etc
If you want to supplement (and you don't need to) then organic soy infant formula thickened with a bit of baby cereal (kind that says add milk) is ok
At least 2 or 3 sources of water in the cage are very important and at least one source should be a water dish that can not be tipped - young rats often look like they are getting water out of water bottles but their tongues are not strong enough to move the ball bearing so they can't actually get any water.