The "Personalized" checklist is proforma'ed (lightly) based on prior year return.  If you have more questions or documents to request from the client, you need to click on Edit and use the Add Item button.

So I can only use the document list I made (in my template in Link settings) for new clients?

No, for brand new clients (and even for existing clients), you can select Individual Template, which is standard and not proforma'ed in the slightest sense.