Rat hierarchy is complex and rich with potential complications in friendship, so I always recommend keeping at least 3 rats at any given time. There will come a time however when you want to be adding in new rats, and here I hope to help you do that!
The Carrier Method:
I recommend this method whole-heartedly.
Starting with a tiny cage, like a hamster cage with nothing but a water bottle attached to it, start here. Scatter food so no one is hoarding the bowl, and be mindful no one is guarding the water bottle. Keep a pair of thick garden gloves nearby, or a spray bottle or a small thick towel to throw on the rats if a scuffle or fight breaks out, but ideally you should be watching body language and interfering before it gets to a dangerous rat-ball fight.
Take as long as your rats need, until they’re calm with each other and napping together in a cuddle pile. Once that happens, you can move them to a medium sized cage so they have more space to stretch out and not be on top of each other. The general rule I adhere to is every time they all nap together without a quarrel, they earn a reward — hammocks or toys/enrichment.
After a few days (again, as long as you believe your rats need to be fully comfortable with each other; I suggest 4-7 days but it could be longer) in the medium cage, you can move them to their permanent cage, again stripped bare. Nothing but clean litter & substrate, and a water bottle. Again, you’re scattering food, and again you’re ready to intervene. Do not rush this step! Once a day, add a CLEAN hammock or enrichment (ropes, baskets, etc), and put them in somewhere different to what your old rats are used to. Watch them closely, and if everyone’s distracted and getting along, if they nap together preferably in a cuddle-pile, the next day add something new. Be prepared to take things away if there’s bickering or guarding of resources/toys/hammocks/what have you.
Ideally in a week or two, but sometimes much longer, you’ll have a cohesive new group who get along harmoniously together.
Buck intros do not have to be as scary as they seem upfront! Ideally, you should have bucks with good temperaments who are open to accepting new friends and roommates, and who won’t show aggression towards other male rats. All rats have their own personalities, but the ultimate goal is for them to jive together harmoniously. The myth that bucks over 6 months old should not be introduced to other adults is just that: A MYTH. I’ve done several carrier-method intros that have gone beautifully and a handful that have been more challenging, but at the end of the day, they’ve all eventually made new friends except one- and he was a half-wild rescue who decided he was better off as a solo rat — an exception to the rule rather than the norm.
Introductions shouldn’t be super stressful. Be prepared to take the time to do them properly, listen to your rats’ body language, and GO SLOW. Do not rush the process, and your new rats should integrate into your mischief beautifully.