are great chess players great at checkers ?

Yes there are some.  And also some checker players great at chess.

Join the club here for info - https://www.chess.com/club/checkers-and-draughts-enthusiasts

Most do specialise at one or the other.  Both games require study and commitment.   

Ivanchuk is a great champion of both, but it is rare to be good at both. One game rewards strategic thinking, another rewards calculation of long variations. 

Yes when I play the 2 games I play them separately.  They require a different type of thinking.  

There's video of Ivanchuk doing a chess and checkers simul, and winning brilliantly in both. I'll see if I can dig it up. He has said that he plays chess professionally and checkers for fun. 

Yes I’ve seen him in videos playing checkers.  Russian checkers. 

Checkers has regional variants.  Russian is 8x8 with flying kings.  

I honestly couldn't tell you without looking it up. 

Suspect after watching him play he is a recreational checker player.  Checkers has more serious devotees.  And GMs.   Like chess I suppose.  I know a couple of checker GMs and it is very difficult to be one.  

Some of us play chess recreationally and some more seriously.   Similarly for checkers.