It is not advisable to mix crayfish (yabbies) with fish. Crayfish are predators that will catch and eat small or slow moving fish, fish that are large enough to not be harmed by the crayfish will probably view it as dinner.
lol, If you got a fast fish that doesnt sleep on the bottom of the tank, put them in their!!
Yabbies are bottom dwellers, meaning they eat on the bottom. Hey you dont need fish in there anyways! yabbies are awsome to watch. If u wanna breed them, obviously get a male and female. And chuck in a heater, about 20 degrees maybe 2 more.