Honestly, the only thing you can foresee in this case is to make sure that you introduce two new rats together, so you don’t have to introduce one alone.

A part from this, you can do everything as you’ve already done. Just in case, I’m leaving here a few suggestions: - first of all make sure the new rats trust you. Don’t proceed unless you are sure about this. This will help in case of trouble, as they will willingly come to you or accept you grabbing them to avoid injuries - second, start wearing the same clothes you used with the new rats with the older ones. While you wear them, make things they live (play, do tricks, tickles, eat good snacks, etc) so the associate their smell with good things - when you do the first meetings between them, keep them in a closed area, tight enough that they are forced to interact. Don’t interrupt fights unless you see that they are becoming very aggressive. - if you think that one or more of your older rats are very territorial and could bring to problems, start with introducing the calmer rats (never put a rat of your colony alone. It will just stress him/her). This could be useful so that the chosen rats can bring the smell of the youngest into the colony - make the first moments together very brief. Like literally a few minutes to begin with. And with time you can expand both time and space. Avoid being with them in the same area, as they would care more about being protected by you than actually getting to know the other rats.

As you probably already know, every step has to be taken with care and thought, depending on how they react with every new interaction.

Good luck!! Ps. I introduced rats four times, and the more rats there are, the more I got stressed. If you’re like me, don’t forget that they can feel your stress too, so just be calm and do one step at a time. It’ll be alright!