It’s a good idea to have the aveilim go to the bimah, so they can synchronize Kaddish. As far as speed, I agree with LC, although each shul will have its standard.
In one shul I’ve been to, the minhag used to be for each to recite Kaddish from his place, until there were two aveilim who were at different speeds, so the Rav asked them to go to the bimah, and they were able to say Kaddish in sync.