Don't get the 5/16" one; it'll be way too long after a short time, and in my personal opinion, all that movement that it'll do will cause more harm than good. Good luck on getting the bump down and getting the 1/4" one.
I would be really nervous using cortisone cream on an irritated (and fairly new) piercing, although I have to admit that I HAVE in fact used it before, but that was only on piercings that I've had for 10+ years that flared up for no reason (as far as I can figure, stress caused them to wack out on me). The 'newness' of the piercing would scare me re: the cream. Did you discuss that with your piercer?