I was digging through some old quotes, and I found this old famous bit from an interview with Metzen the key bit now This means the Kalathi had not been completely exterminated! Where are the kalathi now, and do you think they want revenge on the protoss? A Kalath-Dominion alliance against the protoss perhaps?
who knows how bad "virtually decimating" was. their planet could have been bombed to a post-apocalyptic nuclear wasteland... and set their technology back a thousand years.
The OP pretty much told you everything about them - The SCwiki has an article on them, but aside from passing mentions, virtually nothing has been revealed about them. With as little knowledge about the campaign as we have, I suppose there's no reason to rule out possible involvement in SC2. However I'd expect it to be minimal, like the Broken/Lost Draenei in WC3 (they appeared in - what, 2 missions? and only as extra units IIRC)
Kalathi I doubt Blizzard would have invented the kalathi without putting them into the game somehow. Although they might not appear as units. The Mothership story gave them a "unit" called bloodguard. Maybe Blizzard has some artwork somewhere.
