Hi there!
It's definitely not a good idea to mix guinea pigs and rats. I run a guinea pig/small animal rescue and have loads of experience, so trust me when I say they should not live together.
Personally I don't think it's a great idea to even let them have play time out of their cages with each other. Rats are fabulously friendly, but they are SO different from guinea pigs in their body language, methods of communicating with each other, etc. that they can seriously hurt a guinea pig (and a guinea pig could hurt them! Those teeth are quite big compared to a little rat's body!!) without even meaning to.
They might even get along for a short period of time, but all it takes is for a gp to move the wrong way too quickly, and a ratty can react in a split second in fear, resulting in a seriously injured piggy (and vice versa).
As for living together - first off the housing requirements are completely different - ratties need lots of space to climb and explore, while guinea pigs need tons of surface area space to run around in.
Their diets are completely different too as Jorats mentioned - guinea pigs are strict herbivores and need tons of hay and pellets made to meet their specific dietary requirements, and rats are more like omnivores and need their lab blocks which guinea pigs can't eat. Giving either one the other's diet could result in a nutritional deficiency or even death.
Not to be a downer, just wanted to let you know why I feel the way I do. I would definitely get 2 ratties right off the bat, that way you know they're going to be happy and healthy together. Also think about getting your guinea pig a gp buddy! Guinea pigs are extremely social animals and really thrive when they're in the company of other piggies.
Here's some more reading about guinea pigs and what they need socially:
http://cavyspirit.com/sociallife.htm
I just really don't think it's fair to the little critters to keep them away from their own kind if at all possible. Imagine living on a deserted island with only a crocodile for company. You'd both have no great way of communicating with each other and it would get pretty lonely pretty quickly. That's how I think of little ones living with other species - whether it be bunnies and guinea pigs, rats and guinea pigs, or whatever.