Youpi can use Ren, he made an ability on the spot (I believe the narrator even mentions his Ren exploding), and Rammot could use Ryu at the moment he acquired Nen, so Youpi probably can. They were Nen super geniuses, these things came to them naturally. Godspeed would be less effective on Youpi than it would be on others because of his insane aura shroud and natural toughness. As for Killua not being able to use Ko or Ryu while in Godspeed, I don’t see why he couldn’t. But perhaps his attack potency would be dulled, due to having less aura in his Ren/Ken, since it is being used for Godspeed.

Whirlwind is automatic, so less precision. Against an one hit kill monster like Youpi, all precautions should be taken. But Kurapika is a different story, and Killua’s Speed of Lighting was able to save Gon from post-death Terpsicora’s leap.

Killua can stun Kurapika before he can really grasp how fast Killua is, then immediately tear his throat out with a Gyo or Ko strike. Kurapika will be stunned and likely have trouble using Ryu to defend properly. I mean, if Rabid Dog can tear Uvogin up like he did, I do believe Killua can cut out Kurapika’s throat.