Berndt
I did manage to get a Pro account set up in the paypal Sandbox (not a terribly intuitive process) and received my API credentials. When you create a sandbox account you are issued two test accounts - one for a seller and one for a buyer. You need to log in to Paypal Sandbox using the Seller account. Log on to developer.paypal.com then click on "Test Accounts" under the Sandbox heading. Select the account that begins with "seller" then click the "Enter Sandbox Test Site" button. Note that you will use the password associated with the test account, not the password associated with your developer account. Once logged in, go to My Account, click on "Profile" (took me an embarrassingly long time to find this - it's under the row of tabs on the right), then click on "API Access" under "Account Information". Fill out the info, jump through Paypal's hoops, and voila - you're in business. Note that you will also need to set the options in "Website Payment Preferences" under Account Settings. The only tricky bit in here is the "Return URL" which should be set to http://address-to-the-root-of-your-dashCommerce-site/paypal/pdthandler.aspx
Having said all this I just now noticed the last line of your post. The answer to that is "no". You can use the Paypal Standard provider as well. In fact, I have not yet been able to make the Pro provider work - Get a 404 when clicking on "Proceed to Checkout" with Pro set as the provider. I have, however, successfully processed payments using the Standard provider.
Hope this helps,
Matt