Luffy will accept them as subordinates, it fits perfectly with the story line in preparation for the war against the yonko.
Also, this allows the pirates we loved in this arc to still be apart of the Strawhats without being a new crew member. I think oda played this very well. For those saying luffy will turn them down, how do you figure that will progress the story?
Big Mom, Kaido, and the marines have thier own subordinate crew members as well, so it would be asinine for luffy to to decline the offer and think he can still rule to sea's with only 10 crew members.
He's is crossing his "Prep for being a Yonko" checklist off:
Claim a territory - Check
Have Allies/subordinates - Check