I am by no means an expert on the river. The reality is, prop or not, you should use extreme caution because of both thin areas and submerged hazards - take someone with you that knows the river or go with a guide first. Either way, always wear a life jacket.
I've been from the confluence all the way up past the hatchery in separate jet boat trips - I grounded out once on a missed turn in the confluence because of low light, it was not fun.
The video here shows a run from the High Bridge ramp on the Snoqualmie up through the confluence and up the Skykomish. If the date of the shooting is correct (9-5-2010) then the flow was 2000 CFS. That would be on the low side for any boat in terms of flow.http://youtu.be/nqalHZxUKtg
In this video, there was one area (probably avoidable by taking a channel to the right) that was thin between Ben Howard and Sultan but you can see that its likely possible to put in at Sultan and have a great day on the stretch below and above. It was shot at 7000 CFS in late June so keep a close eye on the water levels. This area works well because its near the hatchery. http://youtu.be/6nmxxhvzNHI
When I look at the flows on the river, I look at the flow from the Gold Bar Station for consistency sake (no tide influence). I think with a prop, you want to run in the 6-8K CFS range to be safe while still having good mobility to fish a variety of holes. As the season progresses, it will move you lower in the system because when it runs at 2k in August/September, you are better off in waders!
I hope this helps and have fun!