Just thumbing through the new WDF and just struck by the awesomeness of GK! Though I am little confused. Are witchhunters the guard arm of the GK or a totally different thing? I'm looking at getting a small 1500pt army in October and so far GK and Space Wolves tick the boxes. If I can use some of my guard then it's a win.

Witch Hunters are the Ordo Militant of the Ordo Hereticus - nothing to do with the GK. GK are part of the Ordo Malleus, whose prime purpose is to hunt down Daemons. The Ordo Hereticus tend to hunt down heretics - unsanctioned psykers and those who speak out against the Ecclesiarchy and the Imperium.

There's no way to use guard and Space Marines together any more - the only way this could be achieved before the release of the GK codex was to use the Codex Daemonhunters and use a GK force with inducted Imperial Guard. The GK codex has removed the inducted and IG and all allies rules. Space Wolves can't use allied IG either, and it wasn't possible using either the DH or WH codexes even if people tell you otherwise (a lot of players missed the rules that only allowed inducted IG when there were no Allied Space Marines).

What you could do is take a GK army along with Inquisitor Coteaz and a few units of Inquistorial Warbands, make them up with warriors and equip them with guard issue gear. You could take a elite Inquisitors but that just allows 1 warband per Inquisitor, Coteaz lets you take up to 6 warbands as troops - although this reduces the numbers of troops slots for GK units too.

Cheers for clearing that up!