I havent played Halo wars in a while but me and my buds were pretty awesome at it and (the main reason at least) the reason being was this. We used scouts. After collecting as many of the credit boxes scattered nearby I would send my warthog (or ghost if I was a covie) to see what the enemy was building and based on what they made I would prepare a counter. If you are facing a covie, it is a good idea when you are scouting to find where their leader is and to see if his base is locked and making units.
One should note that scouting can result in the death of your scout hog/ghost and resources may be wasted to replace. Enemies may also stop what they are doing once you scout them so keep a constant watch on them. If enemies put up turrets to stop u from scouting them, this isn't necessarily a bad thing because they now have that much less money to bring up units.
Rushing actually is a strategy because it takes alot of micromanagement and coordination. A good rusher is always going about trying to get as much resources as possible so he can get his rush going faster. with my buddies on Halo Wars, we used rushes to set the enemy back several minutes which then allowed us to build up our forces while they were still recovering. Scouting your enemy and countering him plus killing his scout unit and or leader ,if possible, to stop him/her from jump starting their rush is a good way to stomp someone's rush. Harassment is also a tactic used to deter someone from getting their forces up. I mean think about it, if 5 hogs with machine gun turrets can make you waste about 1,000 creds who's really winning here?