I'm sure that however you replace or modify skins in games like Unreal Tournament 2003, you can do the same for Republic Commando. It's based on the same engine.
I doubt they will include 'modelling applications' with the editor, though, so as Gabrobot suggests, you'll probably have to scout around for some free modelling programs like Maya PLE.
AS for the character selection, I'm not sure it will be so restrictive as just 'commandos and trandoshans'. I would assume that a lot of models from SP could be played in MP. Hopefully that will include the Wookiee.