Hey, I just thought of this...but aren't cardinal tetras the ones that sleep on the bottom of the tank in an odd position? Wouldn't the crayfish just eat those off while they sleep? Anyone that knows more about cardinals have an answer?
And yes, if your water params are like that, your cycle has barely started. Don't make the typical "jump the gun" newbie mistake and add more fish right away, you'll really just end up with more trouble and kill some of them anyways. Besides, it's way more fun to be able to go to the fish store every weekend or so and get just one or two new fish.