The tax rules in PowerStore are very basic and are setup based on the state that the user selects if they select US as the country on the checkout. You can make use of eCart to alter tax rules or add in your own but this is not a supported feature of PowerStore and will be custom.
If you choose to make changes to the tax rules in PowerStore you should make sure to look at this thread with updated files first:
As for using both Authorize.NET with PayPal you can only use one at a time, not both. You can change which one to use in the admin area for PS3.