Enable plugin for specific posts

If they have the same url structure. For instance https://www.thedomainyouhost.com/somepost/post1, https://www.thedomainyouhost.com/somepost/post2, https://www.thedomainyouhost.com/somepost/post3. Then you can set a plugin filter for the url https://www.thedomainyouhost.com/somepost/ and select also affect children. If they have different urls then you’ll have to set them individually either through the page edit screen or with filters.

What about specific pages like only having nextgen show up on nextgen gallery pages?