Assuming the squads are set up as one elite/brute and 7-10 grunts:
When all the grunts in a squad are dead, the almost-powerless elite pulls out a sword and charges (basically to be shot down)
When the elite/brute is dead, the grunts get a significant rate-of-fire decrease, health automatically decreases
Give Spartans some type of increase in attack or special attack against scarabs
Also assuming you cant just fire the MAC gun whenever you want, give a reason for why you cant use it again (many RTS games have special powers that have to unrealisticly "recharge" and your left saying why?) ex. its shotting at other things, the barrel has to cool down, etc.
Allow marine squads to jump on to scorpians and ride them around
The only thing I have to say about all these grunt sayings is that they are very high-spirited until they are actually getting shot at so all these things about moaning when you tell them to go somewhere isnt very realistic.
Since grunts are supposed to create a funny feeling, they can do this by being all happy and loyal during training and marching, but when they get to the battle its hilarious when they turn around and run away crying