Well, you can have marines, just keep the marauders and hells coming, what happens is, maras take a hella long time to kill, and hells deal the real dps, so you push maras ahead
and bring hells up the rear, any marines are just bonuses, but marines are far inferior to lings with metaboost. Thors are also great tanks for absorbing banes and ling damage, while hells do the real dps.
Trick is to micro hells to keep them behind your wall of tank units.