I think the tightness of the edge and sts may be from doing the CO incorrectly somehow. To get it a little looser, don't pull on the yarn tight, especially the thumb loop, and leave spaces between the sts. It occurred to me too, that you may be knitting the first row between the sts somehow, instead of into the loop on the needle. Usually that would cause the old st to just fall off the needle and unravel, but you might be knitting into the purl bump and the st on the needle, which would be very tight. I think you're just not doing a step right on the cast on somewhere.