Congrats on your first tank! You have great natural aquascaping skills!
If you want corals you should test for calcium, alkalinity, and magnesium just before you start to add corals. You don't need to test for those until then. Be sure to use high quality test kits for these three parameters, like Salifert or Elos.
A protein skimmer is optional on any size tank, but is a great help for novice reefers, so I would suggest you incorporate one if possible. It will help to buffer the mistakes you will likely make. You need to buy a quality skimmer though, as the cheap ones do very little. The Tunze Nano skimmer is one of the best skimmers you could buy for your tank, and conveniently fits into one of the chambers.
Don't worry about the diatom algae right now, it will probably end up covering most of your tank for a couple weeks. Once your ammonia and nitrite levels hold 0 for a week straight you can add a small CUC to help get rid of the diatoms. Personally, I prefer snails to hermits. Keeping your lights off will help with the diatoms as well. There is no need to have the lights on until you add some critters.
You may find some useful information in the links in my signature as well.
