!. I never kept illegal species on purpose. I've maintained species that were legal when obtained but, became illegal. Those were destroyed or sold off to keepers in legal areas. I did get into a lot of trouble when I relocated from the NW to the Gulf area in that several of my species were illegal in the south even though I bought them legally in the NW. It was that event that prompted me to generate the Illegal Species Lists - By State that is maintained in the Marketplace stickies and, in fact, been seen all over the web.
2. I've maintained several central filtration systems and still use this type on a 16K gallon system. The tanks are separated by UV and the tanks still use individual pond-size sponge filters. The rest of the hatchery was on sponge filters run off of a single commercial blower.
3. The best advice I can offer newcomers to the hobby is to learn as much as you can from as many sources as you can. Joining a local Aquarium Society would be a huge boost in gaining personal experiences of a practical aspect. I joined the NJAS when I started out and it brought me tons of info and woke me up to species I never dreamed existed out there.
4. The best advice I can offer new Monster keepers is to build first, stock later. I hate to see keepers struggle with any of the monster species due to starting off with an inadequate size aquarium or proper filtration system. The problems that may come up seem to amplify when there is a lacking in any par of the husbandry needed in keeping a lot of the unusual and large species out there.
I'm afraid I must bow out of the remaining questions since it's really a personal-preference issue in what fish a person maintains. I don't go for the fad-of-the-month club like I've seen too many times on this and other sites. I always recommend keeping species that intrigue, interest, and fascinate the one doing the keeping. By keeping what's popular usually leads to poor maintenance and poor care because the keeper never built a proper bond with the species to want to offer it the best life possible.