The trouble here is that another group of schooling fish wont fit in your dads 16g tank. You have suggested another 3 zebra danios, which sounds about right for the capacity of your tank.
Your other option would be a different fish, that doesnt get too big and would be ok on their own or as a pair.
Maybe a smaller gourami, like a dwarf or honey gourami? Or a ram?
Edit: Zebra danios can live in a temperate aquarium, but im assuming your tank is heated.