Hi, I've had a nice pair of redclaws for a few months now.
They are about 5/6 inches long at the moment and should grow to 10/12 inches I think.
I had them in with my fish to start with and they did leave them alone for a short time.
After they had settled in to the tank they would nip a passing fish at feeding time.
As time went on they got a bit more assertive. I never saw them chase a fish,
but I did see one of them get a parrot fish in a head lock when it got a bit too close.
I put them in a 20G cycled tank after that. The female is carrying around 50 babies.
There are some pics in the invertebrates section.
You will need several tanks, (as I do) if you go down this road.
Check out crayfishmates.com for more info.
Good luck, Don