To answer your first question, we have to know how many tetras you already have.
Male live bearers such as platys sometimes become aggressive when deprived of the company of females. However, you have to have a higher ratio of females to males or the aggression can become worse and they may just horndog the female to a premature death.
What is your PWC
Often times new aggression can be traced back to the other fish becoming unhealthy due to unclean water conditions, especially if they are in the early stages of fin rot.