Fish only systems are just as simple as freshwater tanks. You are just making it harder than it is in your head.
1. If you have a HOB
filter it will do. It isn't best, but will do. When you want to reach a different goal to better something in your tank, we can talk other things like ATS, refugium...whatever.
2. If your heater is submersible then it'll be just fine.
3. Yes you need a powerhead. Just a small one will do, you are just looking for CO2
4. Tanks under 30 gallons do NOT need a skimmer. You can easily do larger water changes to address parameter issues.
5. Cycle the tank? Get a cocktail shrimp and tie it in some panty hose. Walk away. See you in a month!
6. Check what the sand is. Some people use pool filter sand...I think aggregate based is the way to go.
7. You just need porous rock. Macro, base, live...whatever fits your budget. Get one lbs per gallon. Same for your sand. Good to go.
8. If you aren't doing coral, then light doesn't matter. Put whatever makes you happy there.
9. The shrimp goby/pistol shrimp combo is a neat relationship. Might not see them much, but never seen it in a small nano sized tank personally. Just have to make sure they will bond up. Not all goby and not all pistol shrimp care about one another.
10. Saltwater isn't hard, so stop thinking it is. Get yourself a refractometer. One of the best items in your toolbox. Accurate salinity compared to a hydrometer.
EDIT: Just saw you say you have ro
. You win the saltwater lottery. Less algae issues. I run my reef off of distilled water, but I'd be much better off if I had ro
with my TDS
at 0 like I did when I started it.