Yes, you can have any combination of subscriptions
Free, Pro Monthly, and Pro Annual subscriptions can be applied to any of your sites. Each of your sites is able to have a different subscription designation.
To upgrade a free site, navigate to the Pro tab in your dashboard while viewing that site in your Sites List and select the subscription that fits your needs.
🌟If you subscribe to additional annual subscriptions, you will be subject to Apple’s prorated billing structure. Read more about that here.
🌟To see a list of all of your current subscriptions, you can view them with Apple here.