timeline and how it relates to imported fbx animation

I have a looping animation that is imported via fbx, frames 1-1200, works great. Now I need to keyframe some other elements within touch, I have successfully used the keyframe COMP to animate the visibility of some objects. However, I need things to happen in sync. I cannot quite figure out what is happening, but I would expect if I set a key on an object to be visible at frame 300 an off again at 900, they would stay in sync. I have tried the global timeline toggle, but it seems to have no effect. Obviously I am missing something fundamental to the way the timeline works now, if someone could shed some light I would be grateful.
MD