Does anyone use vintage lenses, like the old Canon FD lenses(with an adaptor) or any other old lenses?

I'm planning to get the c-100 mkii
i own a 24-70 2.8 previous ver. I know that the aparture is digital hence the jumping and snapping on the image while closing and opening the aparture on DSLR. Is it the same thing on the c100?

is there any way to have the aparture control feel like in video cameras on EF 24-70 smother like in video althogh it doesnt have an iris ring?

Vintage lenses that have stops at each aperture setting need to be "de-clicked" to make them quiet and smoother. But still, they work in f-stops. A lens designed for video is silent, smooth and adjusts between f-stops. It's one reason DSLRs are a step backward.

ok, thank you.

Can a EF lens be mounted via adapter with an iris ring on it? Just to have this ring(i have no idea if there is anything like this out there).

Never seen such an adapter. If you really want declicked aperture check out the Samyang cine lenses - very good value for money.