When Fuzzy Rat was adopted at 3 weeks old we also had a litter of mice, my daughter thought it might be fun for her to play with the mice so she put them together... Fuzzy Rat pretty much just stole the mice's food and ignored them... When we set up a play area for the mice Fuzzy Rat would jump in and steal their treats even when she was older.
Our part wild rat saw a mouse after she had lived outdoors for about 5 months.. she attacked it on sight and I got a nasty bite blocking her from killing it instantly.. Max would chase wild mice around the house, but she was a pudgy bred for meat rat and never actually managed to catch a mouse, so I don't know what would happen if she caught one...
So yes, some rats are OK with mice, but other rats will see mice as tasty treats or as nifty toys.
Bad things can happen to your mice very quickly, so the only way to find out what will happen to your mice is to see if your rats kill or eat them... guide yourself accordingly.