Me and a buddy of mine are trying to figure out if the idle screw affects the mixture at all....anyone know?

The idle screw is the set point at which the throttle barrel is open, it does effect the mixture in one way, it determines how far the low speed needle sits inside the spraybar. As your barrel opens, weather it's slide or rotorary, it will open up the clearence between the two, hence the mixture changes. That setting is the base air setting, how much air the motor requires to run at a minimum speed, idle.
The othe screw is the low speed needle, this regulates how much fuel is added to the minimum throttle setting (air), when you tune the motor you should start rich on bolth settings and adjust the motor for the best reliable base idle. Now move to the high speed needle and lean it to achieve the best top speed of the engine, but not too lean, you want to back it down from peak a few hundred RPM. the last step is to start dialing in the low speed needle, What your looking for is a smooth transition from idle to full speed with no burbling/hesitation (rich) or sagging (lean), an easy way to get you in the ball park is to do the pinch test. Simple enough to do but requires good listening skills, take a pair of needle nose pliers, and while it's idling, pinch the fuel line near the inlet pipe for about 2-3 seconds and let go, what your listening for is the motor to speed up slightly and return smoothly, shortly after releasing the pliers. If it doesn't speed up at all, then it's too rich. If you pinch it and it drops off and quits right away, then your too lean. Open the needle 1/2 a turn.
This should get you close to the point you want to achieve, you may have to lower you base idle (air) after doing the high speed needle, and remember that the low and high needle are a ballance between the two as the barrel is opened up. so if you richen one, you'll have to lean the other. little adjustments are the key, avoid wholesale adjustments when possible!

So say you have your mixture set correctly. And you increase the idle. Will that make the low speed richer or leaner?

My understanding of how it works is this

The low speed needle isn't in a fixed position, relitive to the throttle body, like the high speed needle is.
As you add air (throttle body opens) the needle opens also letting in more fuel, so the needle setting (air/fuel ratio) stays consistant wheather your idle opening is at .5mm,.75mm or 1mm.
As you open the throttle the lsn mixture setting is the same, just the volume of both increase.

So yes the mixture changes, they both increase, but the ratio stays the same

Is that the answer you're looking for?