Yes, when I am clicking (+) button, sometimes Quick create form is opening & sometimes Main form (but not always the first from the list of Main forms, I sorted them using Main Form Set) is opening. We have tested only for Accounts and Contacts and only using (+) button. Quick create is enabled and for these entities is available.
On the Quick Create Contact form we have a loop-up to Account (Account Name) and on the Quick Create Account form we have a loop-up to Contact (Primary Contact).
My testing scenario: I receive a Mail in Outlook, press Dynamics 365 symbol, then select Track and finally (+) button.
Could you pls help me with docs about Outlook CRM App describing what forms are being used there and possible limitations / rules.