Can patches of moss be placed in the cage to help with humidity?
Yes, use dampened Sphagnum moss. Can also be placed in hides to create "humid hides" during shed/whenever.
Do I have to cover the sides of the tank? The tank would be in a extremely low to no traffic area.
No, but it's easy so you might consider it. I have three sides covered in insulating black foam to help keep the heat in.
Do heat pads melt the tank?
No. Your heat pad MUST be regulated by a thermostat. A good option is a Hydrofarm, a high-quality option is a Herpstat. The thermostat will keep your heat pad from getting too hot. Your heat pad should never reach a temperature that could melt plastic or crack a tank.
Any tips on keeping humidity?
The easiest ways to keep humidity are to limit air-flow and use a high humidity substrate. Cover the top of the tank with tin-foil (as suggested in the tutorial) and use cypress mulch or dampened cocohusk. I personally prefer cypress mulch mixed with a little bit of aspen because I don't like how dusty cocohusk can get.
Also, should I invest in a 20 gallon tank instead?
A 20-gallon will probably be fine for an adult male. The general rule is the enclosure width+length should = the total length of the snake. So a 20 gallon could hold a snake up to 3 feet. However, I personally think you should invest in a PVC enclosure by the time your snake reaches adulthood. They are expensive, but your snake is going to live a LONG time, so you have time to save up for something that is better than a 20 gallon aquarium!
Do I need a heat lamp and heat pad? Ive heard they don't like overhead heat.
An under-tank heat pad is highly recommended (don't forget the thermostat!). Your snake is on the ground, so you want to heat the ground. You may also need a supplemental heat lamp to increase the general temperature of the whole tank to 80 deg.
Seriously, read the tutorial that both I and SKO linked to. It's a really strong recipe with everything you need, for people who plan on using a tank for their ball python.