I have never hard of this being done via a "Y" harness, but technically I suppose you could, but it is much more common and more flexible to mix these surfaces via the transmiter. The problem with using the "Y" is that yopu would not have rudder control anymore on the transmitter since you are basically plugging the rudder into your aileron channel. I'd strongly suggest running this mix via the transmitter. I assume by "prevent yaw" you saw ont he tape they said "prevent ADVERSE yaw?" Adverse yaw is the tendency for ailerons to cause a yaw in the opposite direction of the roll, which causes the plane to slip thru turns.

I have done this, it really doesn't effect takeoffs unless the mix has a large volume, and if anything, it improves the aileron only charasteristcs of the plane when set correctly. You can also of course just fly coordinated, i.e., put rudder in when you need it.