First welcome let me be the first to say "Welcome to the Forum". Its good to have you here. There are many people here on this forum that own White'e so you have a lot of company as they are very good "first" tree frogs. I'll try to answer some of your questions but please make sure you check out the car article to the left it really does cover it all.
1. I personally don't like to buy frogs from Petsmart or any of the chain pet stores because it seems all too often they were improperly cared for or come with diseases. Then the new owner spends their first few weeks with their new frog trying to get it better. That's not the best way to start out in the frog hobby. There are many quality breeders in your neck of the woods where you could get a healthy captive bred animal and it would be a lot less of a headache. They change colors based on temps, humidity, stress, day, night, color of their surroundings, etc. See the threads on color changes. Although there are ones that do lean toward the blue phase.
2. That size tank would be fine for two.
3. I've never had a growing moss so I really don't know. Sorry.
4. With babies I would feed as often as they want and as much as they want. Use a cricket dish so you don't have loose or missed crickets running around the cage. Dust feeders daily.
5. With adults you have to worry about them becoming obese. Main diet should be crickets dusted each time. They are hardy eaters and will overeat if you let them. Some people feed their frogs once a week others feed them 2-3 times a week. I personally just keep a visual eye on my frog's plumpness and if they are looking too chubby I cut it back. They should never be so fat it starts to hinder their mobility.
6.Frogs are nocturnal so the lighting requirements will pertain more to what kind of plants you will want to maintain.
7.Whites do great in pairs.
8.You will need to bake any wood brought in from the outside to kill anything. Bake it at 300 degrees for an hour.
9.Make sure you get yourself an accurate temp and humidity gauge. I prefer the digital ones they seem to be more accurate than the dial ones. Depending on the humidity level in your home you'll probably only need to mist once or twice a day. Check the guages and adjust accordingly.
10.You can use tap water just pour in some water conditioner that you can get in the fish or reptile section to remove chlorine or any other chemicals that can hurt them.
Good luck and post some pictures of your setup when you get a chance.