I ran the MFS last summer, launched on August 12 with around 550 CFS from Boundary Creek. We flew our coolers and a few dryboxes and the majority of our beer into Indian Creek and picked those up on the morning of day 3. This is highly recommended.
It was BONEY, but I would do it again in a heartbeat at that level. The best advice I can give is to run your tubes very, very soft. If you can walk on them they have too much air. Some members of my group didn't buy this for the first couple days and they got stuck WAY more than those that ran soft tubes.
You will stuck and hit rocks constantly. Don't stress about this. Just try to pick the best line and the "better" rocks to get stuck on. Keep good boat spacing, have fun, and enjoy the week in one of America's greatest wilderness areas!
Zach Collier at NW Rafting also put together this wonderful and very helpful guide for the MFS at low water:
Low Water Considerations for the Middle Fork of the Salmon River | Northwest Rafting Co.