in ranged combat your opponent can take the models he wants to remove unless you make more wounds then he has models, then you decide one of the models that gets a wound and he gets that save first, then any wounds after that he gets to pick.
in close ranged combat, you dont remove from be back, you remove the units they would be hitting infront. though, it is possible that one of the models in the back picked up his gun in the middle of the fight, hence why he still has his plasma cannon.
should read the rules in your rulebook, they are very clearly stated ;)
and yes, you have to declare what your attacking, so if your attacking his marine squad, he cant take the wounds out of his imp guard squad instead.