I concur with Amanda. It is perfectly true that different Betta can have different "personalities" for lack of a better word, but a Betta is still a Betta and this fish species is "programmed" by selective breeding to be a fighter, and when you programme fighting instincts into animals, anything can happen. The safest course is to keep a male Betta in its own space, a 5g tank works fine.
And the tetra need larger groups; at least 6 but if you have space, even more. And with Black Phantom Tetra, get a mix of males and females (they are easy to sex, see the data in our profile, click on the shaded name). The natural displays of the males is beautiful to observe. Depending upon your tank size, I would suggest 4 male and 3 female or a larger group in a similar ratio.
Byron.