All the stickied (yellow) threads are worth reading in the
octopus care section.
In the group is a list of links to discussions I have saved to recommend to new/future octopus keepers. There is one discussion about tank cycling included. We also have a
Tank Talk forum to start you thinking about how to design a tank set up. The C
ycling a Cephalopod Tank entry has three excellent links to articles to help beginners learn about the cycling process.
As you are starting to grasp, keeping any kind of marine animal involves a whole lot more than just learning proper feeding. You have to learn about a whole environment, how to create it and how to maintain it. Typically, a marine environment will have 3 forms of filtration, mechanical, chemical and biological. The biological being the most important but the other two cannot be neglected. Rockwork provides the substrate for your biological filtration to grow. Live rock initializes the process by including the initial bacteria culture as well as initial food for the bacteria. Cycling the tank grows the bacteria through a process of ammonia->nitrite->nitrate conversion. The bacteria makes this process happen so rapidly that you should never see the first two deadly components. Nitrate, the third and final part of the process is removed through water changes, exchanging new saltwater for tank water every week. These water changes are a necessity as they also take out other pollutants. In answer to your question, you should not put animals in your tank until all of your live rock is fully cycled but you can slowly add new live rock to a cycling tank (one without sensitive animals).
I keep two 60+ gallon tanks octo ready but most keepers only keep one. Often both are occupied so the "backup" concept is only good part of the time. I do have a smaller tank that I could use in an emergency and that I use for small animals and usually don't have all three occupied
. If you want to keep fish or corals, I would recommend keeping them in their own dedicated tanks (corals and fish can be mixed to create stunning displays but it is a long, slow expensive road to creating a mini-reef). I am a strong opponent of keeping any fish in an octo tank. Fish are not the best food for octopuses, can attack them and will pester them.
Sumps can be useful for keeping extra cycled live rock but the primary purpose is to have a separate place for your mechanical (skimmer and pumps) and chemical (usually charcoal) filtration and increase the water volume for the display tank. If you keep live food available, it can be kept in the sump but it will be hard to retrieve and adds to the pollution load. Some people will add lighting and macro algae to create a refugium environment to add to the live rock biological filtration.
What will your son's name be?