Coral only? But fish are awesome!
I don't understand why, but I know a few people who just don't care for the fish, but drool over the corals. Learn all you want, but you will make mistakes. You don't need to be a chemistry major to know how salinity works, but it can be a little hard to get under control at first. That sounded like I belittled you...ahh I don't mean it like that. You seem to know how it all works and what you need to do though which is great. What I'm trying to say is that applying the knowledge is a different story.
I wish you the best. Maybe instead of getting seahorses first, try out some gobies or little fish that won't compete and make sure those do well (after the tank cycles of course). Then you'll have your "test" fish that won't be a huge investment/loss.
I researched a lot before I started and thought I knew a bunch, but I still made mistakes or wasn't consistent, etc.
Cycle can take 1-2 months usually. Different for every tank. Having LR
and live sand will make it shorter.
What brand is the filter? Can you put bags of media that you want? That would be the best advantage for a filter (instead of having to buy specific filters/carbon, etc for it). A good rule for HOB
filters is rated double the tank size, so 36 gallon tank, a HOB
filter for 70 gallons. Aquaclear is my fav. filter since you can put bags of anything you want and can cut up large sponges which is cheaper.
The only thing I think you're missing is a refractometer and accurate test kits.