1)No additional configuration is required in the future. You should not create a new class. You can perform the step of 'classification' (ie adding trigger values to the characteristics in the release class) as many times as you want depending on your need in the future.
2) All of PO, Scheduling Agreement, Contract etc can only be released completely and hence, are assigned to the release object 2. As suggested above, you can create different release groups for each of these and assign them to the same Release Class. To trigger separate release strategies for each of these, use document type as a characteristic.