I would only suggest 4 triple fins, leaves room for other creatues, although its up to you, they can actually grow quite big. Unless you want to be cleaning the crust of the glass lid all the time i would remove it. The only reason i had a little fan was because i did not have a chiller, I had the power head setup so the water output would brake the surface slightly, and be cooled by the fan. I did water changes once every two weeks, and did a single freshwater top up every week. I used straight saltwater from Island bay so im not sure about salt mix, but im assuming done right you should'nt have an issue.
In Summer, because it got hot, the fan was not enough. We have a big freezer in the garage so i just used to freeze salt water in ice cube trays and dump them in. Temp always sat nicely at 12c, which is what i wanted.
Pene, are you diving or snorkling to capture this stuff? If you did, you would be able to catch a lot more than just the basic rockpool stuff. Thats how i got my sea slugs and other creatures.
Shae