Meanwhile Newsarama have posted an interview with the series' writer, Chris Roberson, here's some of what he had to say:
The central idea of the crossover is that both Star Trek and the Legion inhabit optimistic futures, in which humanity has gone out into the stars and made friends. In this story, we’ll follow a handful of characters from both universes as they find themselves stranded in a much less optimistic timeline that blends elements of both.
From Star Trek we have the bridge crew, essentially: Kirk, Spock, McCoy, Sulu, Chekov, and Uhura. Poor Scotty had to stay behind on the Enterprise, since a Constitution class starship only has six transporter pads in the transporter room. From the Legion, we have Cosmic Boy, Saturn Girl, Lightning Lad, Chameleon Boy, Shadow Lass, and Brainiac 5.
One of the primary concerns from the beginning was to pick a group of characters that would make sense standing next to each other. So the Legionnaires we’re using have powers, but nothing too far out of line with what Kirk and company have experienced in their five year mission. They’ve faced down hyper-evolved energy beings, telepaths, beings who can move faster than sight, and aliens who can reorder reality with a single thought. Someone who can shoot lightning out of his hands or make shadows isn’t anything to write home about for this crew.