Hello,
The HikaShop plugin "validate free order" is there to mark a free order as confirmed when the checkout is creating it.
If you have questions regarding that plugin, I recommend you to open a new thread, in the HikaShop section of the forum so the HikaShop support team will be able to tell you more about it.
For my point of view, you need to define your workflow, what you want for your customers.
You will need a custom plugin since it requires some special rules regarding the customer shopping history.
Afterwards, there is plenty of possibilities thanks to HikaShop flexibility ; you can even find a plugin in the marketplace to add a free product into the cart based on a coupon.
What I can tell you is that you can give serials when a specific product is bought ; and it's also possible to attach a serial to an entire order (so, not based on a product).
Regards,