Hi Pat, welcome to the forums.
Here are my best answers:
1) You should be fine with adding a couple at a time of different species. You shouldn't get any extra aggression issues, if anything you should get less since they will both be unfamiliar with your tank. My best suggestion would be to add the most aggressive at the end. With having a sump set-up, if you have a fish that is getting territorial, you can put him in "time-out" in the sump for a day or two, then reintroduce him to the tank, and he should lose a lot of that aggression.
2) Some people use a smaller laundry basket type plastic container that will float in the water. I have never used this technique however, even with a semi-aggressive FOWLR
setup, so i'm not the best source of info.
3) I would recommend a lot more live rock, closer to 150lbs. You should only need one heater, to be placed in the sump. I have that setup in my 210g and it heats the water great since it shoots out the heated water throughout the tank. The only extra piece of equipment I use is a UV
sterilizer, which is nice to have, but not a necessity.
4) Cycle the tank without any fish/inverts. Those should be added once the process is complete.
5) Nitrate isn't okay for anyone. In low amounts it is tolerable, but stressful. In high amounts it is lethal. Your goal should be 10ppm or lower, obviously 0ppm is ideal. Yes, routine water changes is the best method for controlling Nitrate levels (as well as not over-feeding or overstocking your tank).
6) With a sump you don't need to use activated carbon. I do however just as an added source of filtration which helps keep my tank readings low and water clear. But with a sump and protein skimmer you have no need for activated carbon as well.
7) There is a difference. They serve the same purpose (getting more pure H2O), but deionized water is the cheaper and less effective source. I have never used Reverse Osmosis water, but am considering it in the near future. It is the best option even though it is more expensive. Using tap water won't destroy your system as long as you use water conditioner, but RO
water is far better, especially for reef setups.
8 ) It does not use suction to pull the water down, it simply uses gravity. Suction is actually bad, and if you have suction your sump would pull water into it from your aquarium and overflow it in cases of power-outages. Your pump will pump water into your tank until your tank overflows into your....overflows =), and the water will then pour down into your sump. The only thing controlling the flow in a sump setup is your water pump.
9) I THINK that would be alright, but I am not exactly sure to be honest. I am sure someone else will be able to comment on that. However, be sure to put in a ball valve though so you can control the flow of the water in case it is too high.