As far as being overspun, do you mean the singles are too tight for the ply, or the ply is too tight for the singles? Are you aiming for a balanced yarn?
The best way I've found to make a balanced yarn is to save a fresh sample of the freshly spun single self-plied into whatever ply I plan for it; that is, if I want to Navajo ply (a method for making 3-ply), I'd take a freshly spun length, about a foot long, and fold it back on itself into thirds and let it twist up as it wants to. This freshly spun "self-plied" 3-ply shows me how much twist to put in when I'm Navajo plying. Really. (I keep telling myself "really!" as I ply ... it helps!)
Then once it's all plied up and I'm feeling like I've overplied it yet again, I wash it in a sink of warm water, rinse it, wring out the water, and hang it to dry. Amazing! the warm bath lets the single twist work with the ply twist and Ta-Da! a balanced skein. Works like a champ. Really!
(posted by me this day on livejournal\spinningfiber)