do you have lr, and what type of substrate(nad how much are you using), scooters really aren't that hard, just when you feed them(if thereisn't food for them in the tank), you have to get the food to them, we use a turkey baster, and brine shrimp, and litterally shoot the frozen brine at him, some food will shoot back up, and the other fish will get fed too, but the scooter can get food,
firefish are really neat, but either get 1,2 or 6 or mosre, eventually , they will become a problem if you have more than one, unless the are mated and or have plenty of them to spread around the abuse(6 or more, and no one will take too much, they all get the bullied effect evenly) otherwise they will eventually kill all but one off
another neat fish is the blue gudgeon goby, and beautiful, but most gobies are easy and have their own cool qualities IMO
good luck and i am glad to see that you are concerned about overstocking
try searching google for salt water aquariums, then search within results for compatibily charts, that may help you alot in your decision
HTH