1) Do I need a different Manage Subscriptions module for each subscription type, or will a single Manage Subscriptions module display all subscriptions?
Answer: You only need one manage subscription module for each subscription type.
2) I created a few subscriptions before I added the API keys to the Manage Subscriptions and adding the silent postback. The Manage Subscriptions module is not displaying those accounts. Is there any way to make the Manage Subscriptions module retroactive (causing the site to ping Authorize.net for existing subscriptions), or do I actually need to cancel those recurring subscriptions and reprocess them to make them show up in Manage Subscriptions?
Answer: In order for the manage subscription module to display subscriptions the user must register using the Smith Registration module there is no way for the module automatically to pull subscriptions from authorize.net that were already existing before you installed the module. One solution you could try is to import the customer and subscription data into the smith_customer, smith_subscription and smith_payhist tables if you want retroactive subscriptions populated in the module. If you dont want to try the import than your suggestion about cancelling the old subscriptions at auth.net and process them thru the module will work fine also.