Can I write a different policy for refunds, exchanges and cancellations?

Yes. Each tab on Plugins → Return Settings has its own policy box:

• Refund tab: Refund policy
• Exchange tab: Exchange policy
• Cancel tab: Cancel policy

Write the terms for that action and click the save button on the tab. These texts are shown to customers.

For one summary on every product page, use the Policy / Summary tab. Tick Enable Returns / Warranty Summary, fill in Default summary and click Save.