Nami and Tama can beat Ulti next chapter if Oda wants it. They’ve been fighting since 991, Nami’s already “lost” the first skirmish but not been taken out, and it’s very unlikely that their plan will fail and Nami will be beaten a second time, because then our heroine isn’t getting payback against the opponent that’s given her probably the biggest physical beating she’s had in the series. That’s just not happening, Nami is for certain beating Ulti. Page One and Usopp, that might still swap, but the Nami fight is set in stone.
The only thing stopping the fights starting imminently would be a big flashback, and that’s unlikely to happen since we’ve already had the massive Oden one. Like @duckman is saying above, this is the tower of justice phase, where every single subordinate fight was shown from begin to finish in only 17 chapters. And that was including three Strawhats swapping opponents and the bits people would count as skirmishing- Zoro vs Kaku being very longrunning in the background.
If we’re comparing to Dressrosa, this is the stage where Luffy and Law started fighting Doffy on the roof. Only this time we’ve already had our big arc flashback. Counting Law’s flashback, all those fights were done in ~20 ish chapters. Take Law’s flashback out, that was more like 12-13. Law’s flashback was what delayed the fights- with that out, it’s time to begin.
The set up for the fights has been happening for ages now. We’ve already had a load of skirmishes. Starting in 980, we’ve had
-Luffy and Zoro (and Kid) vs Apoo
-BM vs various Strawhats (twice)
-Luffy vs Page One and Ulti
-Yamato vs Ulti (twice)
-Luffy vs Yamato
-Kiku vs Kanjuro
-the Scabbards vs Kaido (multiple chapters)
-Inu and Neko vs Jack (offpanelled)
-Sanji vs King
-Franky vs a Number
-Luffy and Drake vs two Numbers
-Zoro vs Drake
-Ulti and Page One vs Nami and Usopp (starting in 991, Tama showing up in 995)
-Zoro and Drake vs Apoo and Queen (multiple chapters)
-Jinbei vs Briscola
-Yamato vs Sasaki
-Sanji and Jinbei vs Fourtricks and Hamlet
-Marco vs BM and Perospero
-Carrot and Wanda vs Perospero (starting in 995)
-Franky vs Sasaki (starting in 996)
-Yamato vs a Number
-Sanji vs Maria and co (starting 997)
-Jinbei vs WW (starting 998)
-Ace vs Yamato (flashback)
-Marco vs King and Queen (starting 999)
-Luffy, Zoro, Kid, Killer and Law vs Kaido and BM (starting 1000)
Long story short- we’ve had skirmishes. Plenty of them. We don’t need another ten chapters of setup before the fights proper begin. Every main Beast Pirate (except Hawkins) has been in combat, some have been for a good while, and Jack’s the only one (again, except Hawkins) currently unoccupied.
As for the Scabbards, they are not going to take over and take the Flying Six from the Strawhats. Their big moment was 992, where each of them landed a named hit on Kaido. They may well help out, but the Strawhats are the main figures. The Numbers are nobodies, no personality, no interesting fighting style, getting tossed aside with ease. There’s no reason to suddenly start expecting anything from the remaining ones.
Assuming we cut away from the rooftop next chapter, there’s every chance we can see the first of the Flying Six defeated as the final chapter of Volume 99 and if not they’ll start to go down in Volume 100.