I don't like aquariums either. Have you thought of an aquarium (20 gallon) with a wire topper instead? With an aquarium the ammonia will build up quickly, you would have to clean a lot more often, your rats might end up with respiratory problems from that. Imagine lugging that 40 gallon beast to the tub for cleaning *cringe*.
QT for people without another airspace/building (I call this partial QT, which I have to do myself). The reason quarantine is neccessary is to stop any spread of disease or ecto-parasites.
Main reasons for QT (2 weeks minimum)
SDA - highly contagious, airborne rat corona virus - can kill a lot of your rats unless treated very aggressively with antibiotics. Dreaded virus.
Sendai - another bad one
KRV - Killham Rat Virus - ranked up there with the previous 2.
Myco - there are a lot of myco strains and if the new rat has a virulent myco strain, it flares up and gives it to your resident rats, they could get very ill or possibly die.
Ecto-parasites (lice/mites/fleas) - can come home in infected bedding, wooden objects, etc, can hitch a ride on you if you touch an infested rat.
Soo a complete/proper QT for 2-3 weeks will allow any of these problems to rear its ugly head and be treated before infecting your own rats.
A partial QT (shower/change clothes before touching resident rats or leave the newbies til last at night) prevents only the ectoparasites and things like myco since they are only contagious if the rats are near each other.
Intro's can start after a clean bill of health 2 weeks later. If the rat starts to show any illness, they must be treated, and once they are well again the QT starts again.
Here is a decent article on introductions...
the Rat Report