does jetpack carousel use a rel selector?

Jetpack’s Carousel doesn’t use a specific rel selector. The Carousel needs some extra data, that gets hooked to the core WordPress Galleries thanks to the gallery_style filter:

• https://core.trac.wordpress.org/browser/trunk/src/wp-includes/media.php#L1047
• https://github.com/Automattic/jetpack/blob/3.0.1/modules/carousel/jetpack-carousel.php#L227

As long as you use unmodified WordPress Core Galleries, you shouldn’t have to do anything; the Carousel will be added to all galleries.
If you use another plugin that customizes the output of the WordPress Galleries, the Carousel won’t be added, to avoid plugin conflicts.

I hope this helps.