Hello, I have a bearded dragon too so I can share some of my experience with you.
Bearded dragons do need to be soaked in warm declorinated water once a week for about 20 mins-40 mins. On top of that I mist mine a few times a week and she loves it while she is basking.
Beared dragons hate it when you change up their enclosure so a 40 gallon will be big enough to house one for its entire life. a 20 gallon is only used for babies.
ok, so the reason you need a bigger tank for an adult bearded dragon is so they can have a tempurature flucuation. all reptiles pretty much need a hot spot and cool spot. the cool spot should have a water dish and there should be some sort of hollow log or ramp for them to bask. the cool spot is 85 degrees in the day time and My basking spot is at 106 degrees farenheit which is preferable, but I have heard of people going higher without exceeding 115. Get a night bulb that is heated. (maybe 75 watt?) for night time heat. if you dont then you run the risk of brumation in the winter.
to be honest, this isnt the best place to look for bearded dragon expertise. try the bearded dragon forum. thats where I go if I have any questions or concerns. I hope this helps.
Uromastyx are roughly the same size as a beardie and will need a 40 gallon as well.
Chuckwallas could fit in a 40 gallon, but I would suggest a 90 gallon for a pair
if you want to stick with the 20 gallon and never upgrade then get a leopard gecko. they are cute, hardy, and simple (but not too simple)