Am I allowed to ask questions?
Of course. Why not?
But it doesn't mean he's free to give you detailed answers -- she's a person with her own wants, needs, and limits and their personal life as a couple is their own. Where the "TMI Wall" is at will vary -- but you can talk about where that line will be. Perhaps over time it even changes.
Why do I feel so insecure? Because she is 10 yrs younger? Because she excites him?
It's normal to feel anxiety and weird -- change is change. Until it becomes the "new normal" it will feel different because it IS. It is not the "old normal."
Will she take over from me?
Take over what? What do you fear you will lose?
How do I be a good mono wife to my Poly husband?
Be you the same as always? Ask him what his wants, needs, limits are that you can reasonable meet and meet them. Tell him your reasonable wants, needs, and limits and ask him to meet those.
I'm mad a t myself for feeling insecure and jealous...
Why? Feelings are just feelings that bubble up. They are not logical. Rain is rain. Sun is sun. Emotion is emotion. It is internal weather that will blow on through. You are human. It is ok.
What do you do with yourself when they are out together?
Whatever I normally do? Be with friends. Go out myself. Be home alone with my hobbies.