Hello,
1 - Yes for sure.
The goal of HikaSerial as an add-on is to perform a link with an HikaShop product in order to perform specific action when the product is bought.
So, it is by design an obligation that you need an HikaShop product. And that product can be create whenever you want.
2 - Yes ; that's something we implemented in the first version of HikaSerial Subscription.
When you have several products, it process the recurring details (period and sum).
If you have non recurring products, they won't be in the recurring value.
If you have recurring products with different period ; the recurring will be disabled.
3 - No they are not using a CRON.
Recurring is based on the payment platform. For Paypal Recurring, they platform will perform the payment notification for each order.
Afterwards, it could be possible than some payment plugin would implement the recurring using a CRON ; like for example "Customer + Card" oriented systems (Authorize.net CIM, MangoPay...)
But HikaSerial Subscription requires a CRON regarding the notification emails.
Regards,