The surface area doesn't really matter. It's how much of a bioload there will be. If you just wanted 2 fish, I'd suggest the firefish instead of the clownfish. And most gobies have an easy to moderate care level. Just know the carolina goby lives in coldwater systems so before you fall in love with it's brilliant colors, don't get it! 3 fish should be fine as long as you add them slowly and after your cycle is complete. Yes a CUC would be good to have for eating the excess food and waste and eating fish or inverts that have died before they fully decayed. Often, fish will hide and starve then die and you won't be able to take them out, so it's good to have a CUC that takes care of it before the decomposing waste eventually turns into nitrates. Reefcleaners.com is a good website for ordering and reading about some basic CUC members.
Yes even with LR
you will still need to cycle it. If you have a lot of cured rock you probably won't need another source of ammonia and it could cycle quicker than it would with another method of cycling. If not, say you have some LR
and base rock or all base rock, you need another source of ammonia so you could put a table shrimp in a net in your tank and let it rot or dose pure ammonia.
Freshwater bacteria is different from saltwater bacteria so that won't work..don't add it.