We have heard that you can pick up enemy weapons. We don't know much beyond that.
There has to be some order to it, to address the multiple bodytypes - like if a Light tried to pick up a minigun. They did mention that picking up weapons off the battlefield, is kinda like a last resort tactic, since they will yeild little ammo in them.
As far as swapping weapons, each player gets a primary and secondary weapon, but these can be changed at command posts. Classes are not locked into using certain weapon, so players can use any gun they want, as long as your bodytype can use it.