I am having PayPal enabled on my site. Once it is enabled, will it automatically be enabled on all donation forms?
Yes, existing donation forms will have PayPal automatically added. You will need to edit each existing form if you want to remove PayPal as a payment option. The only time this isn't true is if you have multiple merchant account options when configuring Donation Campaigns. In that case the answer varies depending on a) which option in the drop down is selected and b) which merchant account has PayPal configured as a form of payment.