The fired workers are suing, not ACORN. They are suing because they were allegedly illegally videotapes.
I do not know if this type of case warrants a 3rd party subpoena duces tecum being issued for ACORNs documents. It may take a lot of legal wrangling and will probably fail.
Fired workers are filing the criminal complaint.
ACORN is suing in civil court to stop distribution of the video.