McCoskers are VERY jumpy fish. A sealed top is required for them, speaking from experience. They are beautiful fish, but a tad expensive ($45 is the normal price around me) and just too 'skiddish' of a fish for my taste. Walk anywhere near the tank and you scare it to jumping out.
Randall's gobys are extremely hard gobies to keep alive, there are many other easier options. Yellow, pink spot, blue spot, diamond (for cleaning sand), fusi, just to name a few.
Royal Gramma is a great fish. Firefish on the other hand, let's just say I have tried well over a dozen in my life, never once had one live past 30 days (I know others have much better luck than me). I have had much better luck with zebra or scissor tail dartfish compared to red or purple firefish.
Fire shrimp, forget about it. They don't clean anything really, just decorative, but the problem is you hardly ever see them. They live inside/behind rocks and are very rarely in view. And as you found out, peppermints can be harmful to some LPS corals, mainly favias and acans.
I would not recommend big LPS corals on the top rock, they tear easily with flow and from the looks of your tank there is a lot of flow at the top. A plating or branching montipora would be the best use of space/light. Leathers go in the sand and don't care for much light... or a ton of light.. they can take anything, but they grow big so I would start them off at the bottom and in a couple years they will still be growing out of the tank. I think the best leather is still the neon green toadstool, with a fiji yellow being second.
Personally, if it were my tank, new, and in your position of still learning, I would 1. be patient, you still have a ways to go until the tank is really cycled and stable.. not cycled as in no ammonia, cycled as in no diatoms in the sand (bacteria is built up enough to keep it clean along with maintenance). Once it is cycled and stable I would add montiporas to the top, maybe a spongodes, sunset, and a purple cap to give some variety and color. Middle of the tank I would place LPS corals like duncans, frogspawns, candycanes, blastos, etc.. and at the bottom I would place leathers, mushrooms, zoas, acans, and chalices. Just giving you a general idea of types of corals and placement.