They are, but they have different paths from Killer
- Drake has his own mission, that's why I put him on a different 'category' when compared to other SNs. His rank doesn't necessary indicates his strenght and I can see him fighting Commanders (personally), but as of now he has no business with Kaido since he took responsability for the Live Floor (which is still a big deal)
- Apoo and Hawkins, as much as I like them, have not the same guts of the 5 on the roof even if they are strong. Apoo was an informant from the start, he literally snitched Kidd Alliance so he'll not be accepted as an ally. Hawkins on the other hand trusted his cards only and decided to submit.
All of them can still go to the roof for different reasons but they'll still not be like the first 5.
Talking about why Killer is there: he sides with Kidd and, just like him, decided to fight till the end instead of submitting. He only '''''submitted'''' when Kidd life was blackmailed, and this makes him just a more valuable character (is really similar to what Zoro did for Luffy in Thriller Bark). He's also setting a 'parallelism' with Zoro seen their role in the crew (first mate and combatant) and their fight/interactions.
Killer presence fits perfectly and Oda builded it well, he'll surprise all of you even more in the future.