Jofly has touched on what I really like about my fly fishing these days - total minimalism - I have even got rid of the bad habit of taking two made up fly rods, now it's one rod, one fly, no berley and no other trash or rubbish to get in the way (although I do keep a squid rod handy). I certainly don't catch as many small fish or kahawai as I used to but the fish I do catch are all quality fish.
Re: leaders. Big snapper will bite you off occasionally, but they'll even bite through 30lb fluorocarbon when they get it in the right spot between their teeth. They're not a particularly dirty fighter though so I use 20lb tippet most of the time and 25lb if I'm targetting big fish. Bite-offs I just write-off as bad luck.
For kings I use a short tippet of 20 to 25lb backed up by a long heavy butt section. This is because I find the leader tends to bust further up when a king drags it through the rocks, even hard against the flyline connection.
However, the real secret to fishing kings on fly is to find fish over sand - hence the massive importance of the ray-rider kingfish fishery that has been discovered in recent years.
Flies - search some of the threads on this forum. You'll find that everyone has different tastes, but most people still catch fish. Snapper are actually aggressive fly eaters. The secret is to present the fly in a way that the snapper is not suspicious. A non-spooked snapper that has been presented a fly by a very stealthy angler will eat just about anything.
The best sinking line suits the depth of water you regularly fish, but I like to fish "fly first". So I use a heavy fly on an intermediate line in eight metres of water because a heavier flyline will beat the fly to the bottom. In eighty metres of water when fishing for kings I use a heavy fly plus the heaviest fast sink shooting head line I can manage and work the top thirty metres of water. It can be a very successful way to fish, although sometimes it is a little hard to avoid the rats.
Hope this helps... all good questions.