Cotton is fine...lots of people use it. I used it all the time until I tried rayon for wicking. Now I don't use the KohGenDo Japanese Organic Cotton at all, that I'd bought multiple packs of, with 10 pads in each. If you ever decide to try rayon, make sure it's made of 100% rayon fibers, because they make some that is a cotton/rayon blend, too. You don't want that though.
Even though cotton shrinks when it gets wet, you still don't want the juice wells
stuffed full...so it blocks the hole in the wells even when it's wet. But you do need enough in there to draw the eliquid up, to keep the cotton in the coil wet. Your leaking is probably caused by not having
enough cotton actually
inside the coil...so when it does get wet and shrinks a little...it'll still be touching the inside of the coil loops. You'll get it just right with some more practice, I'm sure.