1. Can the keyboard be assembled with a different switch of my preference?

Unfortunately, they only come with Gateron brown switches. There's a simple reason for it. Offering one switch simplifies inventory management. The reason it's Gateron brown switches is because it's the most requested switch among the earliest customers, so it's stuck. If there's overwhelming demand for another specific switch, it may change at that time.

 2. Can the keyboard be assembled with no switches?

Short answer is no. You wouldn't want it assembled with no switches because they stabilize the PCB inside the keyboard. If there's no switch, it's possible for the PCB inside to shift. Even 2-3 millimeters of shifting can make hotswapping more difficult for you. Although it likely will only take you around 10 - 15 minutes to correct this, I doubt you'd want the headache. The switches are purchased in bulk, so the cost is low enough that it wouldn't change price very much anyways.

3. But I see other people with customized taiko keyboards?

If you see pictures of customized keyboard from TaikoHub, it's because they customized it themselves after they received the keyboard. The keyboard is made to be easy to customize and upgrade.