Epic/patron sets can't take them, yeah. Some damaging secondary set powers can take them though. If you mouse over the power in question in the enhancement management screen, you'll see "Scrapper archetype set" under allowed sets.
Mine is in Dark Consumption or Soul Drain (forget which), on my DM/Shield.
Does it increase the critical chance of the Epic Pool Powers?
If it doesn't, it's probably a bug. Again, no testing to speak of, but I've been noticing far more frequent crits with Fireball using the purple ATO, so I can only assume it's working fine. It's also modifying crit rates through a global buff, and not a per power basis, so I'll put my money on "yes."
Do these have to be slotted in Primary attacks or can they be slotted in your Epic Power Pool attacks as well?