i had a single pair of danios 1 Zebra and 1 long finned leopard, they never got aggressive although having plenty of room might have helped and my tetras woulda fought back LOL but when i moved them to a bigger tank i left the fake rock in old tank and within days had bout 10-15 baby danios all with zebra pattern i didnt even know they were preggas, unfortunately i moved them into the new tank 2 soon and my big black widow gobbled em up, he literally hunted em once he knew they were there while my guppies and danios gave up after 1 chase. wots ideal age to add baby fish like these to a tank with adult fish
also if ya do get more danios try getting a long finned variety they look quite pretty
i also now have another 1 of each danio but these are still young.
but in general u need a few more and they are very lively fish so do need some room for charging round the tank playing all day.