OK, Protien skimmers aren't nessisarily needed, unless it is a big system, or an overstocked tank, or a bio-mechanical set-up with more difficult fish. They are one of those things where they offer cost savings by reducing the consumption of cousumables, like salt. A skimmers job can be done by waterchanges. If it is a nano tank, it will take a long time to "save" the money you spent on a skimmer back, in money you diden't spend on salt for waterchanges. On a large system, the return can take just a few months, if that. Ballence the cost against the benefit and make the desision that is best for you
Live rock is recomended, about 1 KG per 10l of water volume is the minimum recomended, but double that is better
After that, you need lots of flow, arround 20X an hour turn-over through your water pumps, again more is better usualy
With that you are away with your "essential" hardwear, though you can run fish-only systems in bio-mechanical systems, where powerfilters do the job of the liverock. This isn't usualy recomended for reef or invert systems though
I have not used the Red Sea range of skimmers personally, but most members on here that have bought one, have either binned it or sold it on. Apparently they are noisy under-performers, so if you do want a skimmer for your system, you will likely need to replace it. I personally run a DIY skimmer on mine, that is doing a very good job
If you have a spare bucket and air pump and drink fizzy drinks, you can get yourself a cheap but effective DIY skimmer built for minimal £, arround £10 if you already have the feed pump
If you want to keep coral, don't forget lighting. Some coral needs seriously good light, while other bits are OK under "good" light. Lighting needs consideration for coral, so you know what to be looking for, for light units. Supplied tank lighting often wants "upgrading" from the standord...
All the best
Rabbut