Knowing Luffy he will turn all of them down. Frankly I don't want them to tag along with the strawhats every single arc. The dressrossa arc was too messy with too many characters. Not to mention half of the main crew were missing half of the time. The alliance will definitely come back for the final war but I'm not sure about the next few arcs. We need more focus on the crew itself especially the next arc which I guess will be the return of Jinbei against Big Mom.
I think what they're asking is for more of an alliance, but with him being the head of it all. Technically, they're Straw Hats. For example, look at Whitebeard. He had thousands of infamous pirates come aid him with their lives on the life. He was their "big daddy" so to speak, like Bart was calling Luffy. Their crew didn't sail with Whitebeard, they had their own crews. Each crew had their own captain. However, each crew referred to Whitebeard as their father, and labeled themselves Whitebeards.
All they are asking is to be crews supporting his flagship crew. This doesn't mean they will sail with him. Like with Whitebeard, when the "big daddy" is in trouble, they will come aid him. Because he is their father. Does this mean they'll aid him taking out a Yonko? No. Does it mean they'll fly the Straw Hat flag on their ships and aid him in the last battle(s)? Yes.
