RO/DI water is purified tap water by using a RO/DI filter ( reverse osmosis de-ionized )
for a tank this size you can usually purchase this water at fish stores for about 50 cents per gallon. if you do the math, this will add up quickly and it may be wise to just shell out $200 or so for the unit from the start so you can make this water right from home. that choice is up to you. everyday water will evaporate from your tank and NEED to be replaced with plain old RO/DI water as salt wont evaporate. it may be a very slight ammount of water evaporating and may go un-noticed but this still needs to be done. the more water evaporating, the higher your salinity will rise. ( speaking of salinity, test this with a refractometer instead of less accurate hydrometers )
there are alot of gobies that would be a good choice. if you wish to pair one with a pistol shrimp, look at the watchman types. avoid catalina gobies as these are cooler water species. just to clarrify, do not mix 2 species of clowns.
the usual placement of powerheads is either side of the tank. in your case having the filter on one of the back corners and the powerhead on the opposite sides wall facing the filter would prob. be a good start. this may need to be adjusted as spots of little to no flow are found. the goal is to get good flow around your liverock without causing a huge sand storm. ( dont worry, 100 gph may seem like alot, but its actually not )
clowns do not need an anemone for their happiness. infact, they will "host" whatever it is they feel safe and comfort in, which can end up being anything in the tank. i dont think i would keep a nem in a 10 gallon as they will eventually get so large it will be just about all there is in the 10. nems need a reef enviroment, but again having a large nem in a 10 gallon will end up stinging all corals it touches due to size constraints. it is possible, but perhaps further down the road should be considered
i suggest t5s, metal halides ( which in your case would overheat your tank ) LEDs or a combo of the 3. all types of lighting have their drawbacks. research is key.
soft corals over hard to start. mushrooms, zoanthids... are good first additions, AFTER researching their care.
seems like your off to a good start but i suggest much more reading prior to any purchasing.