If there is a friendly between me and an enemy can I shoot through my team mate or must I move to a new line of fire? I'd also like to know if having the friendly fire option on or off will makes any difference.
If friendly fire is on you can shoot through your teammate and damage the enemy, but the damage to the enemy is minimal. Your teammate will take full damage. If friendly fire is off your bullets hit your teammate and do nothing to the enemy.
It's generally a good idea to always move to a new line of fire and never stand right behind your teammate, but a bit to the side. It also helps if your teammate ducks during a firefight so you can shoot over him if you have to.
If FF is off if you shoot a team-mate in the head their head will jerk throwing their aim off significantly; you can see sparks thrown off the head when this happens if you look closely - try it by launching your own server with bots. This is annoying!
Also, it is rare but, it is possible to be killed by the same bullet passing through someone in front of you (and their is a 'magic bullet' achievement for it). As enemy fire is often concentrated on the front guy, getting out from directly behind him is useful if FF is off and essential if its on (someone rushing with a P90/MG on can simply keep firing, killing the first guy and then hitting you before you can react to having a clear line of fire).
I think you would have to know your team-mate very well to expect him to duck to give you firing capability during a fire fight - and then not stand back up too quickly!
In essence a single-file rush is not a good use of people.