Not sure if I count seeing as I've already answered most of these questions for you... but I'll answer them again
No night heat required as long as the temps don't drop below 60 (70's are better for night temperatures)Do I need heat at night time? If so, what should I use to heat?
Anything that they can't eat: I like paper towels but will soon be switching to ceramic tileWhat substrate is best for a juvenile bearded?
A 20 Gallon is good for a Bearded Dragon under 10 inches in length so it should last awhileAnd how long will a 20 gallon long last a juvenile?