Saltwater Newbie

Hello everyone. This is my first time posting on this forum. I'm new to saltwater, but have kept freshwater fish for years. I had two 20 gallon nanos (one long and one high) in the past but tore them down. I now have restarted with a 55 gallon tank that as of now has about 50-65 pounds of live rock, a shallow sand bed, (I can not remember how many lbs it is) approximately 4-6 red/blue leg hermits, a pencil urchin, dwarf cerith snails, a few nerite snails, 5 Margarita Snails, one green reef chromis, and one Ocellaris Clownfish. The tank has been running for ten months now, and I have a few questions to ask. My main question is filtration. Right now I have a regular HOB filter that you would find at a normal petstore, and was not specifically for saltwater. I'm still confused about refugiums and protein skimmers. So far my fish have been fine, except the death of my royal gramma a few months ago(he used to be in one of my nanos). In the not so distant future, I plan on turning my fowlr tank into a reef, so I know I will need better filtration. I've already been looking into protein skimmers and have been planing on buying one once I find a good one at a descent price. I don't plan on keeping any high maintenance corals, and was thinking about mushrooms, xenia, stary polyp, and maybe even some leather (I had Kenya trees, mushrooms and a colony polyp in one of my nanos). I would greatly appreciate if you could point me to the type of filtration and lighting that will be good for my fish and corals. I have one more question, sorry if I'm asking too much, but I'm also curious about stocking choices. I know that I can't get much with a 55 gallon and I'll be limited to mostly nano fish, but could you perhaps tell me what you think would go well with the clownfish and chromis. I have done some research and found some fish that can supposedly go in my tank: firefish, royal gramma, neon goby, flame angle, white banded possum wrasse, watchmen goby, coral beauty, tomini tang (i'm already saying no to that one), Ignitus anthias, blue gudgeon dartfish, blackcap jawfish, assessore basslet, orangetail dottyback, six line wrasse, melanurus wrasse, yellow candy hogfish, indigo dottyback, orchid dottyback, kliens butterfly, and australian longnose butterfly. I was wondering which of those are not suitable for my tank size and which combination, including my current two, you suggest should go in my tank (or even give me suggestions not on my list). I'm almost certain the butterfly fish (too big and not really reef safe) are out. Also, I think I want to add some shrimp or even a serpant star in my tank, and I am not quite sure of the track record of some of those fish with invertebrates (or the track record of invertebrates with the fish for that matter, because I had a coral banded shrimp that I watched attack and eat my healthy firefish in my 20 long):bawling:. I've looked into all the fish on the list but think it would be best to see what people with experience say about them. Thanks.:)