I actually have hard time visualizing why this would be difficult. How many precomps do you have? Can you send a screen grab of what your timeline looks like? What is in the precomps: Footage? 3D text? Plug-in Effects like Element 3D or Particular?
You should be able to just drag the precomps around and put them where you want, and if there is a comp that is too long, you can easily shorten it. If there is one that is too short, you’ll have to increase the length of the precomp and any layers inside it. In theory it shouldn’t be more complex than that, so there must be something I can’t envision that you are having trouble with. Do you have to be frame-perfect with timing? Lip-sync? What is the actual step where it seems to take a lot of time? Finding the exact spot where to put the comp? Something else?