Before you begin, it's important to know a few things about how sequence settings work in Final Cut Pro. When you create a new sequence for your project, the settings will be determined by the Audio/Video and User Preferences settings under the Final Cut Pro main menu. These settings should be adjusted when you first begin a new project.
When you create a new sequence in any FCP project, you can adjust the settings of that sequence to be different from the settings automatically assigned by your general project settings. This means that you can have different sequences with different settings in your project, or the same settings for all of your sequences. If you plan on dropping all of your sequences into a master timeline to export as a unified movie, you'll need to make sure that the settings are the same for all of your sequences. I recommend checking the sequence settings window every time you create a new sequence to make sure your clips stay compatible, and your final export looks correct.