Explanation of Family Sharing iCloud

If they are both individual yes. You each keep the subscription.

If he has a Family Subscription, then you will be asked whether you want to join it. If you choose to join it, your subscription will be cancelled. However the Music Libraries will remain independent of each other.

No, when you join his iCloud Storage, the transfer will be automatic and seamless. you do not need anything besides accepting.

See here: Share an iCloud storage plan with your family - Apple Support

Yes. The accounts remain separate in iCloud Storage, and no one has access to your account. You will get access to a shared calendar and shared photo album, but unless you actually share something, nothing will be visible to anybody else.

However do note, that iCloud Storage sharing requires Family Sharing, which means all purchases you make will then have to be paid by the Family Organizer unless you have iTunes gift card credit on your account. You will no longer be able to use any payment method on file for you account to pay por purchases. Also the Family Organizer, will then know about anything you purchase in the iTunes and App Stores.