We would like to note that all the accounts created before subscription packages were created are able to log in to your site as normal. It means that they can log in to your site without purchasing any subscription package.
However, they (the old users) are able to upgrade to a subscription package if their user group is different from the user group that will be assigned for them when they successfully purchased the package.
phpFox script only supports for Paypal gateway. For another payment gateway, please try to search for third party Payment gateway app on the phpFox store.