So I have been questioning myself about this: when I'm writing and done with a chapter and I am about to start the next one, can I start with a different scene and others characters like in another place? I am writing in third person omniscient. Do I need to have a character's POV ?
Yes, you can. You just want it to be clear to the reader, be consistent throughout the chapter.
Be careful with how many points of view you use throughout your book, as you can end up overwhelming your readers. I had a few readers bring them up in one of my books once. Having a few points of view, and making them definitive and unique can really add to the story. Good luck.
Sure, no problem at all. Just make sure to let the reader know as fast as possible, as they'd have to do some backtracking in their minds if they thought the chapter started with the same characters from the previous chapter.
Start by immediately making the change clear, and you should be fine. In fact, i find it even more interesting to change point of views, as it gives a lot more depth to the side characters and how they see things, rather than focusing exclusively on the protagonist.