I have a pull pull an my rudder in my new 50cc yak. This is my first pull pull so I'm not sure how they set up. How tight should I get the cables? I have them just tight enough to center the rudder now.

Ah, the question of the century: How tight it "tight"?

There's no really good way to explain it accurately - fortunately, you have a good amount of leeway.

The way I describe it is: They should be as tight as they can be without putting any stress on anything (Like the servo mount or the control horns)

Yes!
It depends on the gauge of cable you are using too. I think the stuff they give you in ARTF's is quite light and has a fair bit of stretch. It will tolerate being tightened so that there's no slack in the system and you can pick it like a guitar string. Now, if you can do THAT, then it's a bit too tight, but there shouldn't be a hint of sagging or kinking in the wire.

I have a Flair models Pull-pull compensator in one of my models, but I can't seem to see them any more. Basically, this separates the pull-pull from the servo, and has a spring tensioned arm. If you overdo the tension, it gets taken up in the spring. The servo links to it with a rod.

Also check it after a few flights. I've had a couple that stretched and loosened up.

Having used pull-pull on many large models as well as some small ones, I have found that two things lead to a successful pull-pull system. 1: have a slight bit of positive Ackerman built into the system (this is where the cable not being pulled will get slightly slack as the other cable is pulled off neutral and 2: just enough tension to keep any sag out of the line, keep it as loose as you can. I use pull-pull on both elevator and rudder as it is so much simpler, lighter and more reliable than push rods. It also makes split elevator hook up easier.