1. With such a low bioload as you have you won't need to worry about your water changes so much. It's usually good to get into a regular schedule though just so you know you won't forget. Or maybe that's just my problem
2. If you're using water conditioner like Prime you can treat the water in a bucket as you fill it (helps to mix it together) and then add it immediately. Depending on your water source though it can be better to let it sit overnight. You don't want to add the conditioner directly to your established aquarium.
3. Yes. When you do your water change, save the "used" water you've removed from the tank and swish your filter media in it to remove the solid wastes like fish poop and old food. At some point you will need to change out the cartridge but it's good to make a plan so that you have seeded media ready to replace it. What kind of filter do you have and what kind of media does it take? If it takes 2 cartridges you can arrange it so you only replace one at a time. Do one, then a month or two later you can replace the other.
Your nitrates are actually at a great level and quite low. The presence of nitrates indicates that you have the bacterial colonies needed to convert ammonia into nitrite, and nitrite into nitrate. Nitrates can be toxic at high levels but if you ever do get a nitrate spike (over 40ppm) be sure you don't do a huge water change to bring it down. You can cause a fish-version of "the bends" like divers get when they resurface too quickly, and chances are your fish will die from the change before they'll die from the nitrates. Bring it down slowly if it gets too high. Trust me, I know
