Last time I went to FS they had an ad on the bulletin board looking for instructors. They wanted CFI, 500 given, and pretty low PIC times if I remember correctly - 500 in type or something of that nature. I talked to some of the staff about working there and they told me that the CFI certs were a bonus, but not a requirement. They told me you didn't even need a IGI or AGI to work there, although I thought that one of them would be required.