It's not a bad idea at all.
I'm not sure that it could ever happen, which is entirely because they come from two different time zone and games. As with most debates about this sort of matter (Halo vs. SW or Halo + SW), the biggest factor is technology and how they are uncomparable. Their is also fighting tactics, and leadership values to be considered. Obviously, coming from a more advanced world one might assume the RCs would be the ones to take control. It's quite possible that the Spartans may not like the idea of having them lead.
In my opinion if the two were ever to meet, it wouldn't be a pretty site. I'm sure a fight would break out within an hour. And as we have determined in previous threads, the Commandos would surely win.