I've run it twice, both around 1000cfs, and both in 14' boats. It was no problems at that level. I imagine you'd be able to get a 13' raft down there as low as 500 or so, but that's sheer speculation. It all hinges on your low water experience, and how much you loath getting stuck. I boated the Ark in 2002, commercially running the Royal Gorge as low as 196 cfs, so I have a warped definition of "low water" and "runnable."
As to the 16' cat, when I saw it at 1000cfs it was definitely big enough for a 16' cat. Below that, your guess is as good as mine.
Oh!! Almost forgot- if you have the time, take out further down than 6-mile gap. That takeout SUCKS!! If I remember correctly, it's like a third of a mile straight up a mountainside. The Forest Service has some information
It's a beautiful trip! Good luck!