Updating 20 Gallon SW Tank?

Alrighty. So currently, I have a little 20 gallon nano. It just has live sand with plastic and rock decorations. It has been running since January and had fish in it since February. Everything is just dandy- all of my parameters are ideal and really the only algae is the regular stuff that grows on the glass that I scrape off during regular maintenance. I recently had a little cyanobacteria outbreak but took care of that with some medicine and a few days of lights out. The cause was the phosphorus in the tap that I use and condition. This tank currently has 4 Nassarius snails in it who I affectionately call the zombies, a maroon clownfish juvenile, and a juvenile green chromis. The two fish get along swimmingly. At first, the clown was a little snippy with the chromis but she got over it pretty quickly and doesn't mind him. He tries to school with her- I thought my tank would be too small for a small school and had read horror stories of chromis killing each other all the time till one was alive anyways. But these two love each other- the chromis doesn't leave the clown's side. There was an orange spotted goby but he mysteriously disappeared. Water stayed the same, he wasn't hiding in the ship or plants, and I went through the sandbed with a toothbrush and didn't kick anything up. Davinchi. So anyways, I want to turn this tank into a little reef now. I am experienced enough that I think I can handle it. I want to use live rocks and get some soft corals and some green plants. I will upgrade the lighting from LED. So I need some advise on what types of plants and corals I can use that will stay small enough for my tank, at least for a while. Eventually my maroon will have to be moved to a 50 gallon when she is bigger, but honestly, she is still only about an inch and a half or so and is not growing quickly even with a proper and varied diet. I think she will stay small enough for the 20 for a while. When she needs to move, I will be happy to move bigger corals and plants with her, but not before that. So again; coral and plant ideas are welcome, as well as some websites or information on the types and how to care for them properly. I use the lighting from maybe 6am to 9pm ish regularly. Also, I want to add some more livestock to my 20. Again, I realize that people don't advise more then what I already have, but I want to hear other peoples sides and decide for myself what is best for my fish. I would like some more color, and some different personalities in the tank. Something that will not harm my two lovely fish already but will not be harmed if my clown bullies a little at first. She doesn't bite or even chase that much, just snaps every once in a while. They are both mid swimmers so some fish of different levels would be nice. I have had experience keeping an ocellaris clown with my maroon (didn't know the difference when I got them together... Did a lot of research since them and am much smarter) and the maroon didn't mind the clown. She followed it everywhere. She was even protective of the goby for a while before I got the chromis. So I don't think she will be that much of a problem... She is still timid of my hand but getting a little more brave slowly. She is not vicious, but I believe she prefers to be the boss of the tank with fish that will not be harmed by her but will not instigate. So numbers, types, experiences, and information on ideas of fish is good, so is a fish stocking list idea that I can do my own research on, but something as a guide sheet to go off of. Thank you! :)