Sophie,

I think all editing programs do the same thing. You generally cannot leave spaces inside a project on the timeline.

However, you can at any time insert a clip between any two clips on the timeline. The only problem I've encountered doing this has been where I try to insert a clip where a transition such as a fade already occupies the conjunction of two clips, then the one I'm trying to insert jumps to another junction point.

So I delete the transition, put the insert clip where I want it, and then redo the transition.

You have the flexibility you want, you just have to do it in a different manner than you picture in your mind. It still works out the same and after you do it a few times you'll get used to it.

Sophie, You can use a color board to fill in the space, adjust its lenth to what you want then add your movie clip after that.
drummer grl