ok, i guess you already know thats no good for 2 clowns, so theres no need to say that anymore. What we need to say is how to help the clowns.
for 2 clowns 30 gallons is best, although some members have 2 clowns in 20 gallons (you can count me in since thats what im planning too) 30 gallons is much easier to maintain and the clowns will be happier. By all means if you can get bigger than 30 gallons go for it. Learn more about them next time, please.
As for the first question, ok you have gotten the very complicated equipment stuff, well you dont need all of them. A UVS is not needed in any tanks unless there is an outbreak of a free swimming parasite or free swimming algae bloom. They are not needed for long-term use. You wont need one since you are a beginner too. Protein skimmer in a tank this small is not needed but it is highly recommended since it makes your life easier by lowering the nitrates a bit. Get a nano skimmer, ive heard very good reviews of the AquaC remora series nano protein skimmer, and very bad reviews of the fission nano skimmer. I would recommend getting one since there is barely any wrong from them (the only bad parts of protein skimming that i know of is that it can remove some good stuff like some stuff corals need but you can always add it back in easily). RO units are optional but RO water, to me anyway, is a must. An RO unit is where you can make your own RO water in your house, but you can always buy the water by the gallons at your lfs or walmart or something. So the decision to buy an RO unit is up to you. If you hear of ozonators, say no to them, they are similar to the UVS but are more expensive and are only used by the real enthusiasts with really really big tanks.
As for the filters you have, keep the hob and use it without carbon filtration as these arent used in saltwater much, this filter should be your main mechanical filter. Dont use the undergravel but keep the powerhead. Live rock or something similar should be your main bio-logical filter
Substrate should either be aragonite sand, or live sand. Dont use crushed coral as these trap fish poop and foul water, also if you sift it a bit it will release a cloud of dust no matter how well you rinsed it.
Good starter fish are damsels (highly agressive and shouldnt be in community tanks or reef tanks) although there personality has something to be wanted, they are very hardy and can survive through many minor mistakes that are almost inevitable. You can also look into gobies, blennies maybe, you dont have a large choice of fish to chose from in a tank this small, but dont think you cant have any fish.