Can I set a resume as a requirement for some jobs and optional for others?

Yes, you do have the ability to do this on a per job basis.  You can turn the system wide resume settings to "Hide" by going to Setup-->Job Board-->Edit Settings [for Public Job Board] and then setting the system wide resume setting to "Hide" here:

Then for each job you can select "Edit Attachments" in the jobs question tab here:

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Then you can add the resume requirement for each job here:

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Those jobs that do not require a resume you can simply leave the box unchecked but you will still need to type in the attachment name and any instructions. 

This is what the candidate will see when applying for the job:

Please note that the system will not recognize the attachment as the "system" resume file and will not automatically be viewable in the Resume tab.  You will need to manually set the attachment as a resume file by selecting the resume icon here: