Pet rats, fancy rats or domestic rats as you see here, make wonderful companions. Part of the appeal of having pet rats is the fact that they often get along really well with other pets in the household.
It's very well known that well socialized, handled, friendly rats not only are great with their human keepers, but also learn to get along really well with other types of pets provided those type of pets will be nice to the rats. In most cases, the rats are not going to be the aggressors, they're not going to be dangerous, they're not going to want to do anything more except exhibit their natural curiosity to learn about, smell, probably climb all over the family dog. But they're extremely unlikely to exhibit any negative, aggressive, or dangerous behaviors toward them.
Many people have socialized their pet rats with their cats, with their dogs, with other members of the household. Generally, it's considered to be a good idea to keep rats together as in the wild they're known to be animals that live in colonies or live in groups. So I always recommend that you have at least two just so they have each other to keep each other company.
One important thing to remember is if you decide to adopt more rats you do have to carefully introduce them to the existing colony. Don't assume that just because they live together that the addition of a new rat will instantly be accepted into the group although it is possible to do this. And, in most cases, people can continually expand the amount of rats they want to own by integrating them into the colony and having them socialize and get along really well.
But, overall, there's not a concern with keeping pet rats with other pets in the household. Clients have had pet rats that live with guinea pigs, that are socialized and living in or around where pet rabbits are. They generally don't inhabit the same cages or live in the same spaces, but they can be around each other without any concern for dangerous or aggressive behaviors.
So, overall, pet rats that are very social, very tame, very friendly often get along really well with other pets in the household. It doesn't tend to be a large concern about if there's going to be aggression or danger in having them meet, and even socialize with, and even potentially play with and befriend other types of pets.