One thing I have learned reading these forums, is that every time one person has a problem with a certain fish or combination of fishes, there will be others who do not. Not discounting character traits, there also has to be other factors involved (numbers, tank set up or the ever-present nobody really knows). I think a lot has to do with who is there first and in your case if it is the zebra danios, then you might have a problem. But you might not. If you have your heart set on guppies, I'd say give it a go. In my experience, guppies can be very nasty to newcomers, too, but they get over it and usually end up living harmoniously.
A suggestion for another fish would be black skirt tetras – they are pretty quick swimmers and I do believe they have the same temperament as zebra danios (what do others think?). Plus they are very hardy. Apart from the lovely guppy tails, both zebra danios and guppies are torpedo-shaped, which doesn’t break up the viewing (the only downside I have found keeping them together). Now, with black skirt tetras you have a different body shape, they don’t get too large (2-1/2 inches, just a little longer than the zebras) but they are round. If you have at least 6 of each, they would each play amongst themselves (in theory) and leave the others alone.
