1. Tien was able to stun Buutenks with his Kikoho in the Buu saga. Pretty sure any version after that would stomp FF Frieza like a child. Without Kikoho. Even without that, he stunned and actually subdued Semi-perfect Cell with his Kikoho. That's a strong technique, but not one enough to bridge a gap as large as the one between him and Cell unless he was at least around Frieza's level. That was in the Cell saga.
2. Same thing with Tien here. He stunned Buutenks with his Kikoho. Without Kaioken, that version of Tien would probably kill at least Perfect Cell with his Kikoho, and maybe even kill SP Cell with his Neo Kikoho. Galan was wrong about Kaioken though. It increases all of the users traits, including awareness, physical strength, speed, ki capacity, and likely life-force as well, "for one heart-beat". However, I think it would take at least Kaioken X20 for Tien to do anything to someone like Fat Buu, and even then wouldn't kill him, obviously. Krillin can still only manage to do as much as Cell saga Tien, probably less, since he stopped training.
3. Again, Galan is incorrect. The Kikoho increases in power in accordance to how strong the user is. This is why it actually did something to Buutenks, when it only subdued Semi-perfect Cell. Tien obviously got massively stronger, since humans can't increase their life-force in DBZ outside of using the Dragon Balls, or fountain of Youth like Master Roshi did. In all likely-hood, ki in DBZ is linked very closely to life-force. As in, it can be converted into life-energy, but not used as it, to say, extend one's life. This also brings up the question of how strong immortal beings could make a Kikoho, such as Roshi or Garlic Jr... Meh.
However, it depends on the versions of Vegeta. Anyone he fought evenly with would probably die from a kikoho from him. Not FF Frieza if we're talking Namek Vegeta, but if he went Great ape and used it, he might have a chance, at least if he used Neo Kikoho. I think Super Vegeta could probably kill Perfect Cell though, if he used Kikoho. SSJ2 Vegeta would do nothing more than stall, or maybe even injure someone like Buuhan with it though, sadly. BotG's SSJ2 Vegeta, who bloodied Bills, might actually be able to do serious damage with it, maybe even fatal damage to Bills.