The seats should bolt right in. I had '02 WRX seats in my '97 Legacy for a short while. I think the Legacy/Outback seats had 3 bolts along the inner seat rail, whereas the WRX seats had only 2, so there was 1 hole in the floor pan that went unused. But the 4 main corners of the seat rails should line right up with the Impreza floor pan.
Not sure about the rears, but if the entire rear seat is only $15 it probably can't hurt to try it!