1. The rock won't die if you only have a little, but 25 pounds isn't going to be enough for a 75 gallon tank. You should shoot for around 1-1.5 lbs per gallon. If you use your bio wheel filter, i would take the bio wheels out of it, as they can often cause excessive ammounts of nitrate.
Some of this can be done. Mike is correct, that is not enough LR for natural bio filtration. It is suggested that you get 1.5-2LBs per gallon of water (you don't have to get this all at once). LR is good for hiding places and safety for your critters. You can use some base rock and it will, eventually become live.
About the biowheel, Mike is correct, eventually it could cause huge nitrate build up. While you are starting, I would leave it in there, especially if you are using a small amount of LR. This will help in the bio filtration, as well. After your tank has matured, you may want to remove the wheel, if it is causing problems.
The CC will limit your clean up crew and needs to be vacuumed often, in order to keep it looking nice. I had CC, but decided to switch to sand. If I had it all over to do again, I would go with the sand.
2. I would have your salt water mixed before you add it to the tank with the sand and rock in it, as adding fresh water will most certainley kill all of your denitrifying bacteria in your live rock, thus making it dead rock.
Excellent point, Mike! I premix my salt for at least 12-48 hours. This has helped me learn to mix the SW, get the pH/temp stable or as close to my tank's parameters as possible. All you will need for this is a large enough bucket, PH thermometer and heater.
Since you are starting out, I wouldn't worry about the "sand storm", since you won't have any critters in there.
Please do not cycle your tank with live critters.
Back to the CC/sand. I did this and ended up having to sift out the sand, with LR and critters in my tanks, a VERY big pain in the rear.
