Definitely go with the 7-9 size, if those are your options. Neoprene booties have a LOT of stretch in them, but what you are looking for is something that wont bind up in the boots. I have a pair of 12-13 neoprene booties on my Simms waders (I am going to get them changed out, but am oddly lazy) and they bind up at the heel, sometimes with lots of pain in the tendon. So if necessary, always go with a smaller size that still fits comfortably. I dont know much about the Simms custom waders, but I do know that they will be more than willing to set you up with whatever will make you happiest. And their repair/warranty programs are pretty darn awesome.