Someone else here is probably more qualified to answer your flea questions (thankfully, I don't have to deal with them where I live), but I thought I'd point you in the direction of the Alley Cat Allies
website, if you haven't already checked it out. They have some pretty extensive info on caring for a feral colony: http://www.alleycat.org/resources_care.html#4
If you can't find the answers you need, maybe try emailing them directly: http://www.alleycat.org/response.aspx
Thanks for all you do for these kitties!