I think a good skimmer is very important, and you are planning to get a really good one, so good job on that.
I still think that a 50 gallon tank might be too small, unless you get an O. Hummelincki.
I agree with skywindsurfer that live rock needs lots of flow to do it's work, so if you are going to pack it so tightly, be sure to put pumps in the sump to get lots of flow around the rock, and avoid dead spots and junk settling.
PC lights might be too bright for an octopus, you might want to look around Tonmo to see what people have said about them.
I'm not a big fan of using powerheads for water motion in the tank. I prefer koralia pumps or even better, Tunze Nano Stream pumps. They move more water, and they don't produce such a focused blast as a powerhead. They also use much less power, and so add much less heat to the system (which I care about because I have a bimac at 56 degrees).
When you say "200 gallon filter" you could mean anything. There are a lot of ways to keep water clean, and while just using lots of live rock seems to work best for most reef tanks, I'm skeptical about the prevailing belief that most other filtration methods (bio-balls, canister filters, etc) "have no place" in a marine system. I've kept octopus, strawberry anemone and gorgonians in a cold heavily fed tank for years using a wet/dry trickle filter, and no live rock at all, and it works for me, so just do your homework, and pick a method that will work for you. I think using live rock (with a good skimmer) while expensive, is probably the simplest and safest way, to go, but if you are careful you will avoid the pitfalls involved with other methods of filtration. BTW, I hate canister filters because I learned the hard way that if you wait to long to clean them the mechanical filter could get blocked, causing increased back pressure, resulting in an o-ring failure and a flood. I need a system that is forgiving about irregular maintenance.