You can get around the mandatory update to ServiceDesk 7.5 and still apply the other updates to your SMP. Instead of choosing 'Upgrade installed products' choose the option 'Install new products'. This will allow you to upgrade your suite or individual solutions.
As far as licensing goes there should be no problems with the 7.1 system. However if you use the repository and Workflow Enterprise Management on the SMP you'll run into version issues with Workflow.