Hello Benjamin,
Sorry, I missed that second part of your question. Moving your email to a new host does not delete your Gmail account, you will still be able to use that with their services.
Kim’s recommendation is a fact as Google reads your emails to serve you ads based on their content.
You need a Google Account to sign in to YouTube. A Google Account works across all Google products (like Gmail, Blogger, Maps, YouTube, and more).
On your last question, I have a StartMail account as my own personal email account.