All you need to import is only the plugins in the plugins folder of RPG Maker Action Combat. You don't need to import anything else. Overwriting the plugins alone won't break anything, unless the plugins change how a feature from themselves works. If that's the case, you'll just need to adjust that plugin command from Action Combat in your project. It doesn't always happen, but if it happens, it's unavoidable because the plugins have to grow so they become better and better. Overall, import just the plugins and it's safe.