Against Yoshi and Olimar, any stages with lots of platforms and small room for pivot-grabbing is good. Best neutral against them is Battlefield, since you run no risk of getting pivot grabbed from above them, and that's their main punishing option. Best counterpick, I would either choose a place with a high ceiling, or one that's always changing... Rainbow Cruise makes them both fight to stay alive while we keep attempting to gimp 'em. If Jungle Japes is allowed, take them there since both kill upwards and the water will kill them both faster than any other character, along with killing Olimar's pikmin. If it's not allowed, then CP Lylat Cruise because of the platforms and how easily Kirby can drop off them, then go back up while attacking their shields.
It'll take a bit of getting used to, but eventually you'll get to like both choices. Try and avoid flat terrain (like Smashville and FinalDestination) so he has less advantages, seeing as their main attacks are projectiles and pivot grabs, and you should do fine.