FishyFun,
As far as bioload, you are OK with only those 3 fish. I think the things people are concerned about is that the zebra danios are so hyperactive by their nature that they have a hard time being happy in a tank as short as a 10. They need some swimming room to really be happy. The guppy in there may or may not be OK with the zebras. I have kept them in the same tank before but I don't keep anything as active as a zebra danio in a 10. My QT 10 is for my betta and some otocinclus right now. My other 10 has some female bettas in it. They are quite young and getting along OK. Nothing fast moving in my 10s. My present zebra danios are in my 125 which is 6 feet long. They use all the space they have in there and seem to like having room to stretch their muscles a bit.
As far as bioload, you are OK with only those 3 fish. I think the things people are concerned about is that the zebra danios are so hyperactive by their nature that they have a hard time being happy in a tank as short as a 10. They need some swimming room to really be happy. The guppy in there may or may not be OK with the zebras. I have kept them in the same tank before but I don't keep anything as active as a zebra danio in a 10. My QT 10 is for my betta and some otocinclus right now. My other 10 has some female bettas in it. They are quite young and getting along OK. Nothing fast moving in my 10s. My present zebra danios are in my 125 which is 6 feet long. They use all the space they have in there and seem to like having room to stretch their muscles a bit.