Sounds good to me. The drainage layer you can use hydroton or similar product or even just regular river rocks. As for the 20 Gallon tank I'd recommend getting the 'long' version so you have more floor space and better the thermal gradient. As far as cleaning, if you spot clean as soon as you see them poop and also take out the dirt which made contact with the poop, you should be able to keep the tank clean for many months.