I recently made a tutu for my 8wk old little girl (6wks at the time) and I used one strand.
And it came out pretty full. It stand up by itself.
Far as the waist stretching out, I've heard a number of things when making them ready to ship and of course to get an actual size when making them custom. I hope you can figure it out. I also found a tut for a petti tut through searching FOREVER!!! http://wildflowersandwhimsy.blogspot...-tutorial.html
Here's one for a pixie: http://wildflowersandwhimsy.blogspot...-tutorial.html