Are the sub accounts required to have their own pricing plan or can the

Are the sub accounts required to have their own pricing plan or can they be operated for free?

Sub-accounts do not need to be upgraded.  As long as the sub-account user creates form in the shared folder, those form will be created in the account that shared the folder/forms.  Since the forms are created in what would be the upgraded account, they would be subject to the upgraded limits.