Well ... the very general rule is to have 1" of fish per 1 gallon of water ... but that criterion is not the only one to be considered when buying fish. You must consider the fish's adult size and its temperament as well (for example, is it fast or slow?), et cetera. I believe zebra danios need at least 20 gallons of space since they are VERY fast swimmers and they would feel best if given lots of space to swim. I think they'd feel cramped in a 2.5 gallon tank. There are may other fish that are even smaller than zebra danios and that do not need that much space for swimming.
With the one inch of fish per 1 gallon of water "rule of thumb", you would only be able to have one. They love the company of other zebras, and swim around in schools together. They are very active fish (I know from experience, I have 6) and they use up every little tiny bit of the tank. Know you probably don't want to hear this, but I think you and you're fish would be a little happier if you considered another fish, maybe a betta?
2.5 gallons is too small for zebra danios and cories... cories like to be in schools of 3+ usually so your tank would be crowded with just the cories... I reccomend a betta or a goldfish for a tank so small... I would also reccomend a 10 gallon starter set because they are cheap and come with everything you need and then you could get cories and zebras and have them be more comfortable. does your 2.5 G have a heater / filter?