If Pouf wasn't worried about the wellbeing of the king at the time and could dedicate all his energy to killing the hunters other than Netero he could have done so on his own, just like Youpi and Pitou. Splitting up his clones decreases his raw strength which is why it was hard for us to gauge his strength but he's still as strong as the other guards at full power.
Killua isn't anywhere near as strong as his brother at the moment and Tsubone's motorcycle ability was a lot faster than his lightning currently is, him and Gon need to be down on D-rank (with a seperate rankigh for Gon-san i suppose)
In his current physical state Maha Zoldyck would not be anywhere your SS rank, Netero was able to survive an assassination attempt of his when they were both in their prime, he'd be like A rank about now.
Slap Halk and Nasubi down to A rank, their beasts are probably roughly even but i wouldn't rate any of em as stronger than any of the royal guards.
On that note, bump the royal guards up to S rank with Beyond and old-age-Netero.
This one's gonna be a can of worms but knock Chrollo up to A and post-death Hisoka up to B. Then bump Illumi and pre-death Hisoka up to C. I'm genuinely convinced Chrollo is up there with Ging and Silva as 3 of the current top 5 nen users.
Bump Benjamin down to C. Being able to steal abilities doesn't automatically make him that strong, Leol was able to and he got his ass kicked by a relatively fatigued Morel who had literally just finished a fight against Cheetu.
Goin off pre-death Hisoka's scale, Botobai should be same rank as him.
And for the love of fuck. bump Tser down to A with Nasubi and Halk. He's ridiculously talented and strong given how recently he took up nen, but pre-death Meruem could solo all 14 princes and their beasts.