It really depends on the type of Luec and the type of Auratus you are looking at. Standard Leucs are fairly bold as well as most (not all) Costa Rican (green & black) Auratus. Both are a smaller species and are arboral, though they do stay on the ground quite a bit. You could keep two in a 10g (vert or regular), but bigger is always better. You would do best do go with a 20g and get a group of four (either species) in order to get the most enjoyment from them. They are very active and a heavily planted tank will help them feel more secure so you'll see more of them.
If you are looking for a BOLD species, go with some sort of Tinc. You can house one and it will even beg at the glass for food! Not sure of your location, but many breeders will sell you darts pretty cheap if you are local. You anywhere near Knoxville, TN?
Lisa