I'd recommend checking with PayPal about that. We don't have any say on which credit cards you can accept and how they are processed on this end.
If you are using our credit card checkout page and it turns out that you need to remove the American Express option there is a way to do that.
If so, go to Seller Admin > Edit Payment Preferences and do the following:
-Check the box to Enable Authorize.net
-In the new boxes that appear for which credit cards you will accept, uncheck American Express
-Uncheck the box to Enable Authorize.net and re-enable your PayPal account.