Reptile keeping isn't an exact science. There are some hard rules that everybody needs to follow, but most everything else is just down to what works best for you and your reptiles. If that means running a fan to keep humidity in check, then there's nothing wrong with it.
For example I live in Florida where it is very hot and humid most of the year. Because of this I can't use certain substrates because my cage humidity goes through the roof. But if I lived out in the western US I would have to use a high humidity substrate to even have a chance at maintaining proper humidity.
Unless you see something wrong with one of your snakes I wouldn't worry about the fan. I can't see a fan causing RI as long as your cage has proper temperatures/humidity.
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