Watch them for a few hours to make sure that they don't injure each other. Some fighting is normal, as rats need to establish their dominance hierarchy, so let them do so as long as they are not hurting each other. If you see blood or they are being particularly vicious, carefully separate them. Do not put your hand between two fighting rats, as you are likely to get bitten yourself. Grab one with a towel or by the base of the tail and give him a little time out in a different area before reintroducing him again. Be patient, as sometimes it take several days before the rats are friendly towards each other. Adding a distraction such as a large pile of table scraps can help them learn to coexist peacefully, though make sure there is enough food that they will not be stealing from each other which can cause a fight.
Adding a few drops of vanilla oil or another scented substance to each rat's back can help mask scents and may aid in introductions. You may also want to try bathing each rat to be introduced, as this will remove their natural scents as well.
Sometimes certain rats, especially males, will just not get along with other rats, so you may have to provide another cage if this is so. You don't want them getting hurt. Also, if one of the rats is a female who is pregnant or is nursing young, she will be extra protective and most likely aggressive towards this intruder, so it is a good idea not to introduce another rat in this situation.
If you have a rat that absolutely cannot get along with other rats, it is all right to keep him alone. It wouldn't be fair to him or another rat to have them fighting all the time. Make sure rats living alone get much more human attention than rats who live together, and make sure his cage is filled with toys so he can keep active and not become bored.
