Cristian, can you elaborate on "booked a trip" - do you mean you have scheduled a two week trip to fish in the area or that you booked some sort of guided fishing trip on the Ruby. I ask because the Ruby is mighty small little stream with limited access, but it is a mighty fine stream to fish as well. I just can't imagine fishing it for two weeks, unless you are staying in a lodge with access, then I could see it I guess.
It is an awesome little stream, it is damned and the water below fishes like a small tailwater through flat meadow. It can be tough, it is a series of tight S bends with deep holes on every corner. Great Brown trout water. Since you'll be there for two weeks you'll have to go hit the Big Hole among other great rivers in that valley. September can go either way, end of summer hot or perfect autumn. Could still fish hoppers, BWO hatches, Caddis on the Ruby, streamers. It is one of my favorite places anywhere. I'd check in with a shop in Twin Bridges, these guys seem cool: http://www.4riversmontana.com/
As for rods...you could get away with fishing everything down there with a 9' 5 WT (or 4 wt, my pref). If you have something smaller, say 8' 3 wt would be sweet on the Ruby.