Learn Spanish (Latin America): Uno siempre puede golpear si tiene una buena postura. - One can always strike if one has a good posture.

No, you don't. Once you establish uno as the subject at the beginning of the sentence, you can just keep conjugating the following verbs in the third-person singular. Repeating it as si uno tiene isn't wrong, but dropping the second pronoun sounds much more natural.

If a woman is speaking, should she use una instead of uno?

She can! While uno is commonly used as a neutral 'one' by anyone, women frequently use una when expressing a general statement that stems from their own personal experience. So a female speaker could absolutely say Una siempre puede golpear....

Could I use the impersonal se here instead of uno?

Yes, saying siempre se puede golpear is another great way to express a general rule. The difference is subtle: the impersonal se sounds a bit more detached or objective, whereas using uno feels a little more personal, as if the speaker is including themselves in the scenario.

Why does the adjective buena come before postura instead of after?