Hi there,
I have checked Subscriptio documentation and I would totally recommend against using this plugin for automated payments because what the plugin does is to save the credit card details in your database.
Considering the fact WordPress is one of the most likely to be hacked / attacked platforms, storing your customers credit card details seems like a very bad idea.
However, if you want to use FirstData subscription features, then the entire recurrent payment system of the plugin must be modified.
Please let me know if you need any more info from my side.
Cheers!