My Black Neon Tetra is acting like a bully & my Guppy is bothering another fish, is this normal?

I'm a new fish owner, only a few weeks now. I'm learning, but in the process, I've lost some fish. There used to be 2 Black Neon Tetras, but one died. There used to be 3 male Fancy Guppies, but 2 were eaten (by a little catfish that the store sales woman lied about, when she ured me it wouldn't harm anything...needless to say, that catfish is now gone, I gave it away). So the tetra & guppy are both the only one their kind (I actually only have 1 of each kind of fish in the tank, right now). The tetra has chosen 3 fish to "harr". He darts at them & they flee. My boyfriend said maybe he's playing. I looked at him once & he darted toward my face. So is he being "playful" or being a bully? And the guppy used to hang around the other guppies, until they "disappeared". Now he follows my platy EVERYWHERE. He circles him, gets in his face, won't leave him alone. Like he's in bad need of a friend or something. Is that normal? What should I do? Please be nice, I'm new @ this. Thanks!