I realize great forms can go where-ever they want to, but can they use their powers outside of their domain?

Some, yes (like materialization), but no for many (concealment, movement, etc.).

I don't think there's an explicit list, so the GM has to decide what sounds domain restricted.

They are domain-independent, though a writeup of just how certain powers work outside of their domain would be nice. Sludge engulf is a big one, that toxic critters have too. It suggests the sludge around the character sloshes up and engulfs them. This is well and good in the toxic domain that most of these creatures inhabit. This is a little shakier when they're roaming abroad.

I had always assumed that sludge engulf (or the non-toxic air/water/earth/fire engulf) was the spirit itself engulfing you, not the environment doing so at the spirit's command. After all, the spirit's got to suceed at a melee attack to get it. Is this wrong?