Kaio-Ken can hurt the user's body if used for too long or if the magnitude is large, like x4. But what if piccolo uses it?
Our green friend is able to regenerate, so I think that after intensive gravity training, prolonged usage of KK in x10 or higher should be child's play for him, given his abilities.
There is no way to know, as this topic has never been covered in any official sources. However, since Piccolo's regeneration utilizes focus, ki and stamina, I imagine he would be exhausted afterwards, and the Kaio Ken will be wasted.
I think Tien Shinhan and Yamcha should be able to use it. I'm not sure about Piccolo though because he wasnt on king kais planet that long. Yamcha and Tien however had plenty of time to learn it. Choitzu is to small and and weak to handle the fkrm more then likley. But Tien? Would be badass to see Tien take the Kaio Kin to a whole new level! Kaio kin x40!!!! Goku looks on in shock while king kai is smiling away.
I mean... Goku has done 100 in a movie. I do think that Tien has got some great tricks up his sleeve though for the upcoming tournament. Can't wait to see what is coming up!
I agree Tien Shinhan is a great character. Always has a new look to freshen him up and you know he has to have some new moves by now. But what movie was that? It wasn't the other world tornament was it?