@Solid:
I think he has more association with them than we think, he has been with them all his life, they call him master, and he is supposed to become the shogun of Wano, which would mean he would basically be second in command of pirate nation Kaido wants to create, sure they would attack and capture him on the under the order of Kaido, but they're pirates after all, and Kaido is their supreme leader…
We have already seen some of the beast pirates are integrated with Wano's society, Sarahebi is a teacher, the goons were relieved they didn't have to fight each other after Orochis death, and the pirates and samurai have been seen side to side in Kuri since the start of the arc, sure most of the beast pirates are assholes, but they're still humans, and like we saw with Denjiro and Sasaki, some of them are (were) friends. I reckon the goons who almost got killed by Queen's ice oni virus would happily join Yamato on his adventures...
Yamato and Jack being the same age probably means they grew together in Onigashima, but they never really became close friends, like Law with Baby 5 and Buffalo.
–- Update From New Post Merge ---
@Solid:
Since Wano never seemed to have a navy, and they don't seem knowledgeable about naval warfare or sailing either, it would be great for the country if Yamato leads the remaining beast pirates as Wano's very own fleet, kind of similar to the happo navy, Yamato becomes an admiral, the remaining beast pirates can repent their sins working for Wano, and the wanonians can learn how to forgive and Momo get's his very own fleet he can sail into the final war, win win.
Yeah no.
That's like Nami not joining Luffy at all and leading the Arlongless Arlong Pirates, the people who contributed and acted indifferent to her suffering and dispar, check your moral views pls.
Besides, at least the Gifters are guarateed to end up as Tama's slaves, and if there's still a reformed Beast Pirates now loyal to Luffy, literally anyone can lead it, they follow strenght after all, Holdem, Babanuki, any Tobi Roppo, literally anyone, you name it.