It really wouldn't be overhaul now for both to join. Oda Might be going for the entering the Grandline Vibe, They entered the grandline almost right after Nami joined, and it seemed like at the time the crew was set, and it remained for a bit if you don't count Vivi.
The argument, "Their both Warlords, their to strong" doesn't really matter anymore. Luffy's heritage was shown at Saboady and then relayed to the rest of the word, as far as they(the world) know, Dragon personally trained him and had him attack Enies Lobby and Impel Down to break out Iva and Inazuma.
Hancocks ability seems hax but it doesn't have a solid explanation yet, for all we know her perfume femur and Slave Arrows might not use the targets feelings of attraction, but anyone in a certain area, like how Buggy's powers work. And also the Merrow Merrow: Merrow might be the only permanent stone attack, the others might only be temporary. Oda can fix it without majorly downgrading Hancock, same with Jimbei who has said he is not very good on dry land, but we saw plenty from him. For all we know he has to absorb some water in to his gills and can keep them their for extended periods of time before tiring. Fishman were never completely explained, and the one arc so far where Fishman were the center, it was on an island that they had a base right next to the Ocean.
And there isn't too many in-story issues with them joining. Both have places Oda can send them to if they don't join: Jimbei could join the Whitebeard Pirates if they stay as a crew(which I see hard to believe at this point) Or he could be at Fishman Island protecting it. Hancock, obviously Amazon Lilly.
Now again, a lot of people think they won't join because Hancock has to stay to protect Amazon Lilly, which is a county that lives off Pirating, and is only legal because of Hancocks position as Warlord, and Jimbei who wants to protect Fishman Island.
Both have islands they wish to protect, and that were being protected by a government position, and Whitebeards influence, respectively. Now, Like I mentioned earlier, Luffy has been revealed to the world as Dragons son, but Luffy, in his own right has done many things that look evil to the world. Now, from the Marines and World Governments point of view, they don't know, and probably Garp don't know if Luffy's came in contact with Dragon, but they do know he has teamed up with two of his subordinates.
Now would it be too crazy for Luffy to declare both Fishman Island and Amazon Lilly his territories? Luffy is most likely headed for the Archipelago with Law and Jimbei right now which is right next to Marineford. Now, there are the marines family's there right now, which means at some point there will be Marines their. What if a Ranking Marine, such as Hina, or some other ranking Marine, that is not too high up encounters Luffy? What if it's Smoker, he could once again tell Hancock about her deal with the government, and Luffy could say, "If you lay a hand on Hancocks Island I'll kick every Marines ass" and at the time, possibly Robin on revolutionary ships arrive, reminding Smoker that the leader of the governments greatest current enemy, is the father of the man who just ordered the marines to leave Amazon Lilly alone. And now if word of this gets out, then he could stop trouble at Fishman Island.