introducing a new male rat to replace the deceased

Question
About three months ago, I purchased two male rats- a hairless and a tan and white Berkshire dumbo- from a woman who could no longer take care of them. She didn't have much information about their background, except that the Berkshire was from Petsmart, and both rats were about 8 months old. Recently the Berkshire passed away; I've been unable to pinpoint the reason.
I know rats don't do well living alone, particularly hairless ones, but I'm wary to get him another cage-mate. I don't believe the hairless killed his companion because he had no injuries that indicated he had been in a fight, and because according to the previous owner, they pretty much grew up together. However, I know introducing a new male into his life at his age might end badly or cause him a lot of stress.
Should I wait for a few weeks and see how he does on his own, or should I experiment with finding him a new friend? I don't want to end up with two rats that have to be put into separate cages.
Also, this pair of rats were my first pair of boys. What's the best way to introduce two non-neutered rats? How important is age similarity? Will a younger rat be accepted more easily than an older one, and vice versa? If they don't get along immediately, does that mean they never will?

I greatly appreciate as many answers as you can give me for my many questions! Thank you.

Answer
Hi
I am very sorry for the sudden loss of you little guy :(
Sudden death in rats is not uncommon.  Usually its from something congenital, usually the heart even at a young age.  I know this may not make you feel better, but at least it was sudden and he did not get sick and suffer like so many of our ratties do.


First of all, the surviving boy...is he laid back or does he tend to be a bit on the bossy side?  let me know about him first and this way I can help decide which route to take.