If you look back through the old DEP thread some of the questions were answered on there.
As I understand it applications are put into 3 piles. 1) definitely yes, 2) definitely no, and 3) maybe. The 1's will get a quick invite to interview, the 2's a quick PFO and the 3's will be reassessed after all the 1's have been invited and the numbers still required looked at. i.e BA may want to interview 10 applicants for every post available. If they get the numbers with all 1's then the 3's will be PFO'd. If they don't have enough 1's then the best 3's will get an invite.
As far as interview slots go, more are constantly added. There will be enough slots for those invited. Keep checking the web site.