A 6 weight is perfect for smallies, especially on bigger water like the Delaware, Lehigh, or lower Raritan still light enough to have fun with them and it has enough backbone to cast the bigger flies. A 5 weight will often feel too light for the job, IMO even a 11" riverborne smallie will work the hell out of a 5wt. Having fished smallies with both I much prefer the 6wt, and you'll want it if you hook into a 18-20 inch trophy river smallie!
I use a 9ft, 6wt TFO Lefty Kreh loaded with Rio Bob Clouser line and short leaders. I like Clousers, Gotchas, and Buggers in white (with or without chartruese), black, & brown. Many times I'll run a black/brown & a white or vice versa, tandem rigged.