Which is a better playmate for a baby rat?

I have a 3 week old male rat. Everyone says that they are very social animals and so I plan on getting another small pet for it. I am going to build a giant cage with levels and everything (mainly designed for a rat). While it is small, I want to get another small animal, I use to breed Guinea Pigs for show a few years ago so I know all there is to know with Guinea Pigs. Im new with the rats. I was thinking of getting a baby Guinea Pig around the same age and sex. Since theyre young enough they will have lots of time to get to know each other. I know their diets are very different and thats not a problem at all. Different water too. The cage will have lots of space for the Guinea Pig since they dont really climb that much. Everything will be perfect for the two to live together. But if I get them at the same age around 3-6weeks old (not sure when I will get the second animal) will they learn to get along? Also what else gets along with a Rat (other than a mouse and smaller things). And where are some good places to get cheaper small animal stuff (toys, food bowls, water bottles, treats, food, etc).