Zebra danio's are fast, active fish and need swimming space, and since your tank is a little small as it is, I would not add timid fish like neons. Also they are more interesting and interactive when they're in groups, so adding a few more danios is your best option. Otherwise I'd only add something that uses a different part of the tank, a small group of bottom dweller like cory catfish or something. Nice tanks are about balance and sticking to suitable fish