i have zebra danios, guppies and cardinal tetras living together just fine.
if you plan to get danios, get 3 or more. i've heard they can be fin nippers but if you give them playmates, they will just bug each other instead of the other fish in the tank. they are a joy to watch as they chase each other around.
the reason i like the guppies is because there are a variety of colors and you can choose what color you want for your tank. if you fry, then get females (2 females per male) or get an all male group like mine. very colorful!
the cardinals add vibrant color to the tank. but should only be placed in a well established tank. i only added my cardinals 3 weeks ago and my tank has been running for more than 6 months.
i hope the very best in your tank set-up! fish add life to any home ^_^