Why start at 25 minutes when I want an hour?
Because an hour you abandon at minute nine teaches you that you can't focus. Twenty-five minutes you finish teaches you that you can. Attention builds like anything else — from a length you can actually hold.
Is the Pomodoro timer necessary?
A timer helps, mostly because it makes the end visible. The technique matters less than the closed apps and the task chosen in advance.
What about music?
Instrumental or nothing. Lyrics compete for the same language circuits you're trying to use for writing or code, and most people who swear otherwise are doing shallow work.
Can I use the hour for meetings if my week is packed?
Then it's not a deep work hour, it's a meeting. Move it rather than convert it — the block only holds if it means one thing.