I would definitely consider a bigger cage, but it's true that with cats around you have to be extra carefull with where it is, and that it cannot topple down for example.
I also second the fact that they have to spend some time outside of their cage. That means being prepared for the fact that rats love to chew anything and everything. Mine loved rubber, sole of shoes, erasers, rubber feet of chairs... So nothing too valuable or that you can't replace.
When I had to do stuff and couldn't keep an eye on them constantly I would get a fistfull of dry uncooked pasta, leave then in a shallow dish of water (or under a light box, or in a maze..). It would keep them occupied until they had stashed away all the pasta in my room.
It does mean you're left with pasta everywhere (I once found it in my pillow...) but it won't rot, you can collect it again and re-use it for the rats.
Using wallnuts is particularly funny since they're so big and heavy (I would take them away before they had a chance to chew them, I'm not sure wallnuts are good for them).
Mine always did their business in the same corner, so that I only had to regularly change 1/8 of the litter, and never did anything outside their cage
(but do expect copious amount of peeing tho, that's how they'll tell you they love you. And own you
Read up on commun illnesses and symptoms, a change in behaviour can be quite telling.
I miss having a rat sleeping all snuggled up in my sleeve
, those expletive
are so darn cute I wish I could have some again.