I just started something with Angora Soft, and I HATE it. The angora is woven in very sparsely, so it really just seems like a cotton with a few stray cat hairs...know what I mean? Also, the first ball I used had FOUR knots.
The real key to subsitution is making sure it's the same weight. There's usually a weight (worsted, sport, etc.) and/or gauge listed on the yarns. If you're getting a close weight or gauge, it should work fine. The other thing to consider is fiber content. There's no reason you can't use a cotton yarn on a sweater that calls for wool, as long as you understand that it will drape differently. Wool's got more elasticity, so it will hug and stretch more. When cotton stretches, it doesn't really spring back, and it tends to hang in a more boxy way.
Hope that helps!
ETA: Duh! You already get the gauge stuff.