Clarification begins...now.
The class buff unlocks occur when a character finishes Chapter 2. At that point, that character's class buff is unlocked for all characters of his legacy.
Each class' buff applies all of your unlocked class buffs with a single click. It's not 4 buttons you have to keep on your quickbar. One click, all the buffs.
An example: I have an Imperial Agent that finishes Chapter 2. The Agent unlocks his class buff for my legacy. I now go log onto my level 1 Sith Inquisitor. When my Sith Inquisitor casts Mark of Power, it ALSO applies the effects of Coordination, since I have unlocked the Agent's buff.
Furthermore, to get the full set of buffs, you only need to unlock 4 classes. Unlocking the buff for Agent and Smuggler still only nets you a single buff effect, so you can't stack their +Crit bonuses together. (Because that might be a wee bit overpowered. Maybe.)
Clarification ends... now.