You might want to consider using another program for compositing...

You can create your 3D stuff in Carrara as Quicktime movie files with alpha's
I'm also using Anime Studio Pro 5.6 for composing the alphas and using the camera's with the Z-depth and particle effects within ASP to do all the stuff that would take up more time to do in the 3D app.
Also Project Dogwaffle is another great tool to use to create additional animated effects like adding distortion (heat haze) and particle effects.

I'm just doing a small 30 second blurb for a friend of mine, but you can really make up a lot of time with planning out your project with additional tools.

Carrara 7 Pro, Anime Studio Pro 8, Hexagon 2.5, Zbrush 4.6, trueSpace 7.6, and Corel Draw X3. Manga Studio 4EX, Open Canvas 5, WACOM Cintiq 12WX User