I think you'll have a hard time finding any Greenbacks in the Poudre, especially the lower canyon. I've caught them out of a number of other bodies of water in the area. If you're after Greenbacks, I suggest Rocky Mountain National Park. The park's fishing literature will show you the details of where, and also what special regs apply.
If you're bent on the creeks that feed the Poudre, Roaring Creek is a tributary listed in my Greenback book. There are a number of pools that might hold fish, but I did not have luck there when I tried a couple years ago.
You can get a foot away from trout in certain situations and still get them to take. Best bet, come from down stream so you don't kick any gravel, mud, etc into there faces. Their backs will be turned to you working your way up stream.