Again its hard to know for sure but I suspect JDS might turn out to be the worst style matchup for Rumble at HW(Overeem would IMHO be another bad one). Johnsons gameplan these days afterall is to exchange at mid range and that's playing right into Jnr's game, the only way anyone has lasted more than a few mins fighting at that range with him is via incredible chin(Hunt and Nelson).
I could actually see Rumble having more success vs Cain, cardio, heart and chin would still be question marks but I just see his skill set being much more suited to countering Cain's strengths. The size difference would be less, Rumbles takedown defence would be put to use and perhaps most importantly I think Cain exchanging at mid range is more vulnerable than Jnr if only due to having a human rather than godlike chin. That said I spose you could say Rumble ground game might also be exposed should Cain get him down.