steve - do a search here using- fish cage - as your search words in many of the headings. I'm not sure where the topics were listed. You asked a lot of questions I will address a few briefly here. Maybe I can get the PBoss magazine editor to do a detailed story on fish cages; subscribe and stay tuned.
1. Fish cage fittings and kits are available from several supply houses. Internet search will no doubt give plenty of results. I think a website also may have been cited here too earlier. For kits and parts see www.aquaticeco.com
Buying a cage, expect to pay $150-$450; build your own, depends on size, small one $30-40 (resourcefull minimum) probably closer to $50-80.
2. I build all my own cages which is much cheaper than a kit. I think this site has a pdf file that shows how to build a cage http://aquanic.org
It may take some looking to find the info.
3. Cage size can be highly varible from small 2ft diax3'deep to large net pens. I use small cages abt thesize of a 55g barrel whereas Cecil uses big cages 4'x8'x4' deep.
4. Most any kind of fish can be raised in a cage. I have raised numerous types of fish in a cage. The main thing for a novice
is to buy fish that are already trained to eat fish food. Training fish in a cage to eat artificial food can be tricky or frustrating without some experience. Expect to kill fish in your learning curve. Some fish adapt to cage life easier than others. Some fish have to be crowded at high density (8-12/cuft) to prevent them from fighting. I prefer to work with fish at lower density (3-5/cuft) in the cage to minimize health problems from crowding and water quality.
5. Walleye in a cage for a novice is very risky and difficult. Firstly you will need to find walleye fingerling trained on artificial food. Without knowledge and experience you will have much difficulty trying to raise this species esp starting with fry. Fry to "decent" size in a cage - 3 yrs to 13"-15".
6. Do your cage learning with an easy to raise fish like trout (fall winter) or bgill. Then move on to more difficult species.
7. Catfish can be tricky and are vulnerable to fungal disease esp when cage stocked in water below 68F. They are reported to fight at low densities.