Oh, allow me to say a few things involving my idea for a 40k FPS.
First off, you can't magically change your class. Each team would have a "base" (well... this is just a rough idea, but I think it'll work, at least they'll have a general spawn point), at this "base" you could change classes and/or buy new weapons.
Also, remember, if you change classes, the points spent in that class would be lost... so, though you could change your class to assault marine, unless your team REALLY needed an assault marine, you could might do best to stay a normal marine.
Oh, and you mentioned that I had to many people in this game...
maybe this would have to be more of an super online game... I dunno, but, considering squads and all that (I can't imagine one terminator walking around and all), you'd need AT least 30 people.