When you publish a "ready to sell" product on your shopify store, if you add any additional variants the EA app will not recognize them, and the order will not show up in the app if it includes something it does not recognize.
On "custom products" Yes you CAN ADD 1 variant option during creation process.
Example : If you publish a custom product, you can add "background color", "number of people or pets", "Theme", or other options you may want.
Things you can not do:
add more then 100 variants