Actually I'm not entirely sure why, but if you OTG someone with 6C they will ground slide longer than if you hit them out of the air into the slide. So from j.214B it is impossible(as far as I can tell) to do 6C > 66 > 2C because once they touch the ground they will tech so you cannot OTG them with 6C. But with j.214C when they are sliding they are considered OTG, so if you 6C them you can dash in afterwards and still connect with 2C.
The same applies to 236B, if you do 236B > 623A > 6C while they are still in the air you won't get the long slide, but if you do 236B > 623A > 6C(hold till you barely get level 2) then you will hit them as they touch the ground and you'll get the long slide.
Though timing the 6C is easier depending on the character, holding 6C just long enough for the level 2 charge will work for almost everyone(doesn't work at all on Platinum).