Be sure to take the mandarin back as well. The shrimp will be fine on its own, its up to you if you want to keep it or not, you could always try to pair it with a different shrimp goby. Any pics yet?
I wouldn't add another mandarin. It is a sign of a healthy tank and would think that your mandarin and scooter will catch up soon. Generally when you have a population boom, it is indicative of too much available food.
I adjust mine every month or so, sometimes longer. I wait 5 seconds before checking it. Meaning I add 3-4 drops close the clear piece onto the water. Wait a few seconds, then take a reading. Congrats on your purchase
Punctuation helps. Filter floss is a type of mechanical filtration. It will catch a lot of the junk floating in the water, you just have to replace it often since it can cause nitrates if left in for too long. I change mine every 2-3 days. Why don't you go to the new hobbyist section and read...
Punctuation would be helpful. Have a look at some of the tank diaries in this section. Have a look at spanko, rebelprettyboy, nano-newb, whisk, they all have nice successful nanos going.
I wouldn't put live rock, too easy for detritus and other debris to get stuck down there and rot. Instead put some chaetomorpha and a submersible light and make it a refugium