They'll figure out how to jump 2ft. I had a lovely pet rat who was very skilled at jumping 2-3ft up out of her play tub and into my shirt. It was pretty impressive.

They'll probably sync up their breeding and you'll have waves of babies. Even in a 90gal I don't think I'd have too many rats breeding. Waste builds up fast, and the more rats you have in there the more rats you'll have to wrangle in order to properly clean it. If you can set up a sturdy lid and everything I'm sure it would work well enough temporarily, but as a long term thing I think it would just end up being a big hassle (especially compared to a rack. I still don't know how I managed to go so long breeding rats without one).