If getting a younger leopard gecko, if its a few weeks old and eating good you shouldnt run into any problems. And depending on size id keep a baby in a 10 gallon until its a bit older. Just makes it easier for them to hunt down food. But a 20 long is great for 2 adults so itd be great for one. Plenty of room. Must have belly heat of around 88-92ish and no sand for substrait. I use paper towela peraonally but if your looking for something more fancy id go with slate tile. Must also have a humid hide as mentioned above to help with shedding. If you have anymore questions please feel free to ask!
-Amber