I know you've had a lot of replies to this thread and already decided not to get rats, but for the benefit of others just thought I'd add. Definitely not. As part of the animal welfare act not only do social animals need to be kept together, but animals also need to be kept apart from others where they are solitary (eg Syrian hamsters, or interspecies mixing which can cause an animal to come to harm).
The part of the act that stipulates this:
Rodents and lagomorphs can (and often do!) carry zoonotic diseases, so apart from the very obvious physical harm that animals can do to each other it's in our duty of care to prevent and protect against zoonoses as much as possible. Obviously disinfection and hygiene are a part of this when keeping several species of animals, but physical contact is a very good way of spreading pathogens. To anyone who does let species out together, no matter how well you think you know your pets, and how quickly you think you can split up should a fight occur, you do have a duty of care to prevent this happening in the first place and take steps to prevent incidents - which basically means not putting them in the situation in the first place :wave: