Re: 6wt or 7wt for smallmouth?
You can go either, but your determining factors are going to be the size of flies that you will be throwing and the weight of the lines.
If you plan on using larger heavy or wind resistant flies (say size 1 or 2), you may want to errr towards the 7 weight. It will make for a better delivery system for those flies. If your flies are not that large, go with the 6 weight.
If you use a sink tip or full sinking line, choose the 6 weight for lines as heavy as 225 grains and the 7 weight for lines as heavy as 275 grains.
My smallmouth rod is a 6 weight Z-Axis. I use streamers as large as size 4 with coneheads with sinktips as heavy as 225 grains.