I think that they will join, as Ulti says that she'll do anything for Yamato since she saved her and her brother, and it'd be nice to see some new grand fleet members.

Also Oda pretty obviously set them up for this, so it's probably gonna happen.

Yamato leading the Beast Pirates and Katakuri leading the Big Mom Pirates to join the Grand Fleet

I read it as the grand feet.

^What--

I always saw this as a future possibility. I made posts awhile back saying how they would fit on the main crew a couple of times. Now that Oda is making them the few main characters in the recent cover stories and how Ulti and PO are in debt to Yamato, I could def see them being aids to the strawhats in the future,