My boss has 9 rats right now. She has done rat rescue for many years and has had many many rat in the past. She has always kept them in a wire cage. She had told me that a rat should not be kept in a tank.

If the tank had a topper on it, it would be ok. BUT- if I remember correctly, you can only get toppers for 10 gallon tanks.
No, you can get a topper for any size tank. We have a topper for our 50 gallon, wide tank. Along with 5 other large tanks. You should be able to get a topper for any size tank.