Can I set up different link text or modal header for some size charts?

Yes!

While in general, all the link text or header text are based on app settings, we allow you to customize them on the size chart level.

What you need to do is go to size chart editor, and you should notice a third tab for "Advanced".

After clicking on it, you will notice the "Advanced Options" tab. Here by default size chart is using the default modal settings.

If you uncheck this, you can start customizing them.