Well, I am certain it is a guppy, he looks very much like a juvenile this:
http://www.aquahobby.com/gallery/img...ticulata_2.jpg
The guppy wasn't schooling with the tetras, it was more of a question of the tetras not schooling while the guppy was in their tank. lol.
In with the Danios, the guppy seems fine, it is like he is trying to learn how to swim faster so he can go pester the Danios! I worry a bit though, because the Platy will eventually go back in that tank. It is jsut odd to hav such an agressive solo baby guppy!