I have to say that ffmpeg will take care about the extension of the file you're passing with the file name. In this way you can make a mov or a flv juts selecting ffmpeg and adding the proper extension to the file name.
When you select the extension you're selecting the container and not the codec. The codec is the one that is defined internally in the composed command line that the program send to ffmpeg. BTW it could be a cool feature to have the opportunity to select the codec and the main parameters like cinelerra does.
Almost in linux I can create .mov .flv .avi and .mpg (different containers) with same internal codec MPEG (libmpeg2).
So it is not needed to render to a png sequence and later use ffmpeg due to you can do it directly using synfig, almost in linux. --Genete 10:02, 18 October 2007 (EDT)