I have never tried to use an airsoft gun to scare off small animals, but I've had very poor results trying to scare off racoons and possums using a 400 fps metal pellet bb gun.
So I doubt that any type of airsoft gun would be very effective against a coyote. I've read that paint ball guns can actually work quite well against coyotes and such but I wouldn't trust anything less than a 22 personally.