As for the collar, I'd go with something shiny, like satin. For the fuku, white spandex is used by most cosplayers. I guess I'd go with tulle or something for the sleeves, I don't now what else could be sheer...
Yes, the skirts are gathered. It wouldn't really work out with pleated skirts anyway.
~Work in Progress~
Sailor Moon ~Classic Musical Fuku~ (Sera Myu)
Joyful Knight Candelira ~Human form~ (Zyuden Sentai Kyoryuger)
Maybe: Eri ~Cheerleader Version~ (Tensou Sentai Goseiger)