Partition

Hey guys, I recently got a 30 gall, filled it with 2 female and 2 male guppies to cycle. Firstly, how do i tell how far along a guppy is? Obv the gravid is one way, but i have nothing to compare it to :S i dont know if the current darkness denotes imminent labour, or early pregnancy. Secondly, I had an idea for a partition involving a frame of coroplast (corex) and a pair of tigts (pantyhose to those over the pond) in the middle. Is corex poisonous? Will i need a seperate filter for the partitioned area, or will the water movement be enough? also, can i raise the fry on just crushed flake? I wont have enough time or money to go to the LFS every day to get daphnia, only for it to go through the tights and get eaten by the "biggies".