There are times when cages are necessary.
For the rats, always. It's the best way to keep them safe from the cats. A necessary precaution when you foster both predator and prey. Sometimes it's also necessary to have the cats in cages. When they come in and they are truly on the feral side of semi-feral a large kennel is often the best way to get them accustomed to humans. Then after a while you you open the cage door and let them explore on their own.
Especially when keeping two sets of cats apart I also have my DIY "screen" door. Once they get accustomed to each other I can open that door and hope that the fur doesn't fly. Just like when they start coming out of the kennel they eventually start exploring the rest of the house, one foot at a time until....
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