Properly specced, they should. Warriors and Death Knights get it from baseline abilities (Thunder Clap and Icy Touch) and while Druids and Paladins from talents (Infected Wounds and Judgements of the Just) and hence could in theory not apply the debuff, I've yet to hear a good reason not to get them.

I wonder if the two debuffs stack or if one will override the other.

You could use the Frost Shock instead if they don't. That way, the tanks could start kiting the bosses around, to try and avoid a little more damage (or it will help that aggro heavy dps to run away when the boss is slowed). Or you can go with the Flame Shock (is that the right name?) and add some extra dps to kill him faster so there is less to heal.

I guess this just makes it so you have even more options for different situations, and personally, I love it.

Don't be silly. Earth Shock just got buffed to match the tank abilities. And while all the tanks can (and should) apply the debuff to at least their main target, it's also true that Earth Shocking the guy(s) the tank didn't get can be helpful. Admittedly, for the caster who's sitting too far away for Pestilence or Thunderclap, you'd probably want to start with a Wind Shear, but y'all oughta be used to doing that by now.

Incidentally, unless the mob is immune to nature spells (like, say, rock, poison, or wind elementals) you can debuff pretty much every mob in the game with the attack speed reduction.