Hi,
1) Yup 2-3 of them is fine but always provide plenty of hiding spaces. PVC pipes of various sizes work great.
2) Some people advise keeping the water level low when feeding live fishes so the crayfish has a decent chance to grab them. Personally I fed mine with hi-calcium pellets (very important for their shells!) instead. Frozen bloodworms are good too, can also try small pieces of chicken or fish or other meat. But not feeder shrimp as some carry parasites that are extremely fatal to crays.
3) ALWAYS COVER your tank and keep something REALLY HEAVY on top. They will explore and they are damn skilled at escaping, often crawling up the wires of my filter, commando-style. I had one for 7 months and he escaped 6 times. This was even with a secure lid with a small weight on top (hence the need for a really heavy one). Do not underestimate them, crays are pretty strong!
4) When they moult, leave the shell in the tank. The cray will come back and eat its old shell to replenish the lost calcium. It takes afew days for the new shell to harden so best to remove the guy in case the others attack him. Telltale signs of moulting include: sudden refusal of food, a split in the shell on its back just behind the head, general lethargy etc. Once you spot these it's a good idea to transfer him out to a small tank or bowl and let the process carry on. Take note that some "unsuccessful" moults may result in breaking of legs and claws or even sudden death for the cray.
Hope this helps! =)