David, we do have a whole forum dedicated entirely to Dahlias here:
http://garden.org/forums/view/...
I'm pretty sure that it will take a Dahlia plant two years before it makes a big enough tuber to produce flowers. That's why most people just buy the tubers, to save the time and effort of raising them to blooming size.
You still should post this question over on the Dahlia forum for more opinions on this idea, though. I haven't grown them in years.
If you buy a couple of different colors of those Dahlia plants you saw, and you cross-pollinate them then, yes you might get interesting new colors and flower forms OR you might get flowers that look like the parent plants which may not look anything like what you had bought. This is another plant that you will have to dig up the tubers and store indoors for the winter, btw.
