I doubt the Grand Fleet itself will get more official captains/crews given the narrator stated that the current seven captains will cause a great incident in the future and the crews were specifically those who helped take down Doflamingo. They might get more members to the individual crews, but I highly doubt it's going to be characters who we already know or that many (at most I would think +56 among all the crews to make the 5600 into 5656, keeping the pun).
There's also Usopp's lie of having 8000 followers, but there are so many groups that will likely come help that will definitely push the number over 8000 (unless Usopp gets his own 8000 somehow):
Straw Hat Grand Fleet
Wano (Samurai, Ninjas, and Yakuza) + Minks + Ex-Beast Pirates (this alone pushes it past 8000)
Heart Pirates
Kid Pirates (Kid might not be a full-on ally, but he won't let Luffy get all the glory)
Kuja Pirates
Sun Pirates
Rolling Pirates + Firetank Pirates (Lola will help, and Chiffon won't let Bege not help Lola)
Saruyama Alliance
Galley-La + Franky Family
Revolutionaries + Okama
Newkama
Rosy Life Riders
Various individuals (Johnny + Yosaku, Hatchi, Surume, Bellamy, etc)
And that's just those who are on friendly terms with the crew, there are other groups that I can definitely see getting pulled into the war, like Buggy's mercenaries, Germa 66, and Big Mom Pirates (though Katakuri and/or Pudding might try to convince/memory-shenanigan other members into helping the Straw Hats).
There are also kingdoms (Sakura/Drum, Alabasta, Fishman, Dressrosa) and Marines (Garp, Fujitora, Koby, X Drake, Smoker and maybe Tashigi) that will probably help too since I imagine the fallout of what's revealed about the Void Century will likely push many to secede from the World Government.
I agree with the idea of Momo joining as an Apprentice, he could learn a lot from the crew especially from Robin and Zoro
Learning Swordsmanship from the future World's Strongest makes sense for the man who wants to rule a country known for its incredible swordsman and Robin teaching Momo about the poneglyphs could lead to something with his family and Wano's involvement in the void century
I think learning Poneglyphs from Robin is the only real plot thread left that would be enough to keep Momo with the crew. He already learned from Zoro, and has the Scabbards to teach him Oden's Nitoryu/swordsmanship. But if Hitetsu does turn out to be Momo's grandfather, then he can teach him the Poneglyph language.