I read somewhere that HC gets an attack bonus vs. Rams. Can you defend against Rams with massed HCs?

They deal 2 damage instead of one. It's still not effective. You can look up stuff like this on [You must login to view link] or in the cheat sheets linked in my signature.

Thanks!
I guess you have to defend with melee units or magonels/onagers against them. The only ranged unit which can kill them easily is the Chu-Ko-Nu.
Edit: Forgot about the Mangudais that's the other one.

Throwing axeman/Mameluke

Well, technically they are infantry/cavalry and they deal melee damage.

Yep and as far as I know the trailing arrows of Chu Ko Nu deal melee damage hence why the are good at taking down rams.

Conquistadors also get a bonus vs them.
Normal ones get +4/3/2
Elite get +6/5/4

HCs deal 2 damage to Battering Rams and Trebuchets (Unpacked) but only 1 to Capped Rams and Siege Rams because of hidden armor bonuses