I want to submit some "people" images to SS, Fotolia and Dreamstime. I know I need a signed model release, but do I need a specific release for each site or is there a single generic model release that would be acceptable to all the sites?

You can use generic:

http://www.microstockgroup.com/general-big-6/generic-model-release-t1435/msg12090/#msg12090

Thank-you!