Definitely bring a can of bear spray and keep it handy. Bear incidents seem to be on the rise in the park, and Slough Creek is a prime area. Buy it at a gas station/convenience store rather than an outfitter and you'll save a few bucks. I've not camped up SC, but I have fished up halfway through the second meadow. You will definitely want a backup plan. A storm up in the mountains can blow out the creek for days. Check in with a park based shop on your way into the park to find out about conditions. Blue Ribbon in West Yellowstone is a good one.
Other good back up streams would include the the Paradise Valley spring creeks, the Gallatin, and the Upper Madison.
Even though you are camping in the third meadow, you'll probably want a car camping location for the way in or out. I'd recommend the Slough Creek Campsite right near the trailhead or Pebble Creek, which is just east and located near the Soda Butte. I think you can make reservations for these sites. In high season, you cant' get a non-reservation site past 10 or 11 am.
I fished the Slough in Aug when the spruce moths were on the water. A medium sized tan caddis w/o hackle did the trick.
Good luck!