If it's not an aragonite based sand (made of limestone) then I'd say get one because some sands have an excessive amount of silicates which feeds diatoms. That should be fine, you don't need a powerhead. I heard you could just compare the color cards and use the same test kit, but some of the colors are different on each. A hydrometer is usually under $10. If they have a 10,000 K
rating or higher then I think they should be fine. T5 HO
would be better, but people even use power compact lights for soft corals, so I don't think you'll have a big problem.
No, 2 firefish is too much for a 10 IMO
and plus if not in a pair they will be very aggressive with the same species. What other goby would you want? A variety of different foods would be best like flakes, pellets, frozen food (mysis shrimp, spirulina). You could make your own frozen food by getting shrimp, clams, squid, etc. and putting it in the food processor then freeze it. Fish love it and it's cheap too.