After loosing my time with automator, 
I post a proposal of 50 for the scripter that would make me this simple script, 
(I'm not use to script, I use visual programing and don't have time to learn now):

The script start it-self when computer start-up.

1/ move all files from "folderA" to "folderC"
2/ move all files from "folderB" to "folderC"
3/ delete in "folderC" all files that are more then 30 days old
4/ delete in "folderC" all files that are less then 20Mb
5/ delete all files that are not video (.mov)
5/ empty trash
6/ wait 30 seconds
7/ move files from "folderC" to "folderA"

this script should work on Mac os X10.5 (apple script 2.0.1) and 10.6

all the best,

Thomas Israel