As far as smell what you do and regular cleaning will help but there will be a smell no matter what especially if you have a strong sense of smell.

They can get noisy if they fight or get down to business.

As far as ratio it is correct for a proper rotation (avoid harem breeding) you will need 1 female rat per snake, less than that and you will fall short on production.

I actually run more than 1 female per rats simply because there can always be issues, female not working out or dying etc.