abs55, you really do not have any clear concept about movieclips or library yet! What you are thinking went in the right direction, but is a bit offtrack. First of all, just the number of instances of a mc wont put much pressure on the processor unless you have some code the computes on all of them! Calculating code is the actual processor intensive part.
For example, you can create many bullets and have them be destroyed after sometime, without lag, but try creating a single bullet and let it go off to infinity with some distance checking code in it...and it will soon start to lag.
I dont know your entire gameplan so i cant say anything specific but you should design the level with a tile based approach, such that, you can generate the level as you go on. Also, you can layout the ground mc or such which have no codes in them before hand, and you can also attach the enemies, just dont execute all their codes if they are outside view.
this type of checking will lessen the burden on processor, but still it will be processor intensive if there are too many instances to check through. Thus you should not perform this check every step!
//execute the enemy code
And lastly read tonypa's tutorials on tilebased level creation to build larger yet lag free levels.