I have a rat pup picked out as my final choice for a friend for my (lonely) 6-7 month old male rat. My only real concern with him (the baby) is that, according to the pet store, he is only 4-5 weeks old. He's pretty small and fairly shy at this point, but is otherwise friendly for a baby who has had almost no human contact as a feeder rat. I know that typically during the rat introduction process, you give each of the rats toys from each others' cages to let them get used to their scents, then introduce them in a neutral, supervised setting face-to-face after about a week. Should I get this baby rat, would it be a wise idea to delay the introduction to my existing boy for a couple of weeks so he can grow a little? What's my best course of option here?

Also: Please don't lecture me about getting an animal from a pet store. I know the risks, and there are no breeders or rescue rats within at least hundred miles of me.

First off, I would like to say bless you for rescuing a feeder, and for getting your big guy a buddy! pets question and answers,www.5d2d.com
It shouldn't be necessary to let him get any bigger, as far as the older rat is concerned. Adult and juvenile males are very gentle with pups. I would be more worried about the baby being able to squeeze through the bars of a larger cage. I got a 6 week old baby for my then 5 month old male, and placed them in separate cages right next to each other at first. The large cage my big guy was in was a modified bird cage, and the bars are spaced about 1/2 to 3/4 inch apart. Little guy had to stay in the hamster cage for 2 weeks before he was too big to squeeze through, but I proceeded with the introductions after about a week of living next door to each other. Once they were introduced in the neutral area (bathtub was awesome for this) I actually put the big guy into the hamster cage with the baby for a week, putting a small drop of vanilla on each of their necks each day to make them smell the same. 4 months later, they are still together and getting along fine. The occasional bath helps as well, although you'll want to wait to bathe the little one for another month or so, so he doesn't get sick. The bath dilutes their scent and bathing them together helps them bond through a shared "traumatic" experience. XD Mine just got out of the bath. They are huddled together in their nest boxes grooming each other for comfort. You'd think I had tried to torture them or something LOL pets question and answers,www.5d2d.com

EDIT: I suppose I should clarify, I moved them both into the big 3' tall cage after a week in the hamster cage. The hamster cage is 12"Wx18"Lx18"H. It is not one of those dinky little cages. Go to a few rattery sites, or find some ratteries on facebook. They are experts at introduction and social behaviors. Having gone through this situation as well as studying rat behavior articles, lab studies, etc for months, I can tell you that a couple extra weeks likely won't make any positive difference in behaviors, and may cause them to become depressed because they cannot groom each other, which is the rats' social "glue". If you decide to keep them apart longer, just watch them closely to make sure they are eating and drinking enough, because depression often causes them to stop.

Rat owner, veterinary student
www.ratastic.com

Agree 100% with the above answer

It is true that young and old rats will get along fine, but that is too young to introduce still. I have read that male rats have been known to kill strange rats under 6 weeks (I don't want to test it), but if you quarantine the new rat for at least 2 weeks, which you should, then he will be a perfect age to introduce. But there is nothing wrong with getting that rat, it is pretty easy to introduce a young new rat and an old one because the older one will almost certainly be dominant, therefore there will be less fighting in the beginning.

is there anyway you can get 2, because there may be a small chance that your rat wont like the new one and it will end up being on its own. pets question and answers,www.5d2d.com

my rats i got from a breeder were living with my older rats when they were 5 weeks old (older rats were 8 months old at the time)

your plan sounds good, you should deff get the lil guy, poor thing...it dont wanna be snake food =( it would rather be your rats buddy =)