A medium would *probably* be too large for you, given that I'm a bit bigger than you and I had to exchange my medium for a small. But then again, my cup size is bigger, so of that measurement, more of mine might be...compressable? What measurement is 30" - ribcage, biggest part, or average of the two?
Stepping into it may help stretch it sooner, yes, particularly if you've got small shoulders and didn't find yourself nearly dislocating them to get it on. But you're probably not going to be comfortable wearing it all day at first anyways, especially if you're a small cup size and may not have been running around in bras more uncomfortable than binders all the time, just slowly increase the time in it to get used to it. At least if it's just the boobs hurting and not causing breathing trouble - do check to make sure your nipples aren't bent in some weird position that's causing them to be pulled in a different direction, too, that'll hurt.