Using hydro on pyro is a strong reaction which depletes the affected by pyro status of an enemy and using pyro on hydro is a weak reaction that depletes the affected by hydro status partly. Reactions also have an internal cooldown between being able to trigger another reaction of the same type using the same skill.

I think no characters can keep up with Klee's pyro application so it's likely XQ being the character to vaporize Klee's flames.

Pyro on hydro is 1.5 while hydro on pyro is 2.0 damage modifier, so if you do build XQ he amplifies your damage massively. I'd say making XQ apply hydro and then making Klee vaporize is awkward when using the burst, especially since Klee is about charged attack spamming from what I heard and XQs burst only works with normal attacks. It's probably worth it for XQ e, then Klee vaporize, but you should do some actually reliability testing.

Take all of that with a grain of salt, it's theory crafting, I also don't have klee.