just keep a defensive humvee(with 2 marines and 2 missle squads) and you'll be just fine. They're dirt cheap, very mobile, and lethal with cannon upgrade.
I've encountered at least 10 different kinds of rushes, but none of them managed to break me hard enough to force me to submit defeat. On same token I've tried rushing a couple of times with greact success, so I can only conclude you have to understand how the game works if you want to be able to deal with early agression, or just learn how to scout properly
I've had people try to send infantry and light vehicle responses to counter the mortars, but inf is useless vs mortars that easily smipe them and the vehicles need to be constantly moving to avoid the barrages. It's very easy to sneak a single APC full of inf into the outskirts of someone's base since everywhere is expensive early game. From my 6 or so uses for this rush the only hard counter seems to be someone who was already going for a gaurdian rush in the first place.