One of the things about RTS games that has persisted despite its obvious unrealism is soft counters, and in cnc it goes as far as every unit damaging another at least a little, no matter how heavily armored the target is. So, this means that even quad guns destroy overlord tanks.
Of course, in reality, vehicles do not have hit points as such: a tank is critically damaged when its armor is pierced and the crew is killed or vital equipment like the engine is destroyed. This may happen on the first shot, or the twentieth. A heavy machine gun round like the one fired by a quad cannon usually penetrate armors worn by vehicles like humvees or even apcs like m113 Gavin. But main battle tanks are usually many times heavier than these vehicles: and even their flanks are so heavily armored, that heavy machinegun fire just bounces off. Small firearms, like AK 47, have no more effect on a main battle tank than throwing hamsters on it.
However, soft counters are important: if you need a hard counter to destroy everything, it'll hurt gameplay. Therefore soft counters need to be rationalized.
Take for example, a rifleman squad or whatever is planned for basic infantry in general 2. I suggest that every basic rifle squad is equipped with multiple light weapons: including light anti tank and light AA missile systems. Some soldiers in squads would carry LAW-style portable rocket launchers, which would damage light vehicles quite severely, but only inflict minor damage on heavy tanks. They would not trace their targets, so a fast moving technical wouldn't be a viable target. But because missiles like LAW are scarce and ineffectual against infantry in the open, the squad would use their assault rifles to attack infantry and soft-skinned cars, and only use their rocket launchers to attack tanks, bunkers and apcs and the like. Airplanes would be vulnerable only to AA missiles, but stationary hovering helos could be effectively engaged with small arms.
So that's your basic infantry squad: they'd have a wide variety of weapons, they could do a little bit of everything, but they would not be especially good at anything. Instead of engaging tanks with assault rifles, the squad would use light anti tank weapons: which at least inflict some damage on even heavy armor, thus making soft-counters more plausible.
Same would be true for specialized infantry squads:
for example, an EU anti tank squad might be equipped with javelin style anti tank systems, which would be a very hard counter against heavy armor, but they could be used as soft counter against infantry since they'd also carry their faction's basic assault rifles.
For GLA, their basic rebel squad would tote AK 47s, but they'd also carry molotov cocktails against tanks. These could be upgraded to RKG-3 anti tank grenades and then to RPG 7s. Not all of them would carry such weapons: only like 2 in 5 members of the squad, enough to inflict some damage on tanks so that they can be used as soft counter to a tank rush.
In summary, the point is that whenever soft counters raise doubts about plausibility: like hamster throwing catapults destroying overlord tanks, include secondary weapons to explain these away. The hamster catapult should have an anti-tank missile as secondary weapon. The secondary weapons should not be super-effective, but enough to justify the minor damage that is required to make it a soft-counter.
It would be cool.
It would obviously require more work to make multiple attack animations and stuff like that for many units.