I'm sure their "costumes" will be like the Justice League's costumes are on Smallville. That is to say, leather.

Alexz World, A fan site devoted to singer/actress Alexz Johnson, confirms that the actress is set to play Saturn Girl Irma Ardeen in the upcoming "Legion" episode of 'Smallville'
The site's author writes, "...the rumours that Alexz is guest-starring in the long-awaited 'Legion' episode of 'Smallville' are true. She is filming this episode as she awaits the final mix of what may be the final song for her album."
The episode, written by comics super-writer Geoff Johns, will introduce the Legion of Superheroes to the show's mythology.
I'm sure their "costumes" will be like the Justice League's costumes are on Smallville. That is to say, leather.
She may be 22 but she looks like a child. Anyway, typical CW, Smallville 90210, cookie-cutter casting. Call me unimpressed.
How about if they "INTRODUCE" Superboy(man) into the show first, hmmmmmmmmm?
Well, I guess Supergirl (Laura Vandervoort) finally gave her "Instant Star" costar some hollywood work. Good for her, and Alexz is actually an ok actress- from what little I saw of her.
The Legion of Superheroes are basically kids, so she fits the role perfectly.
According to the Spoiler page at Kryptonsite.com, The Legionaires will be wearing civillian attire, but the color scheme will match their comic-book counterparts uniforms, just like Clark's blue shirt and red jacket.
Had to search for another pic on the web where her face is visible. Looks nice. And 22 years is not too young. I was afraid they might cast teenies for the trio from the future. I wonder if they are in costume.