Gruit

So, I recently decided to try a few Gruits. At least three: 1)A Traditional Gruit, with the traditional three herbs. 2)A Spruce Gruit with a grain bill similar to an amber ale. 3)A Black Licorice Gruit in the style of a sweet stout.

So my questions are:
What yeast should I use for the first one (I was thinking a Belgian Trappist)?
For the second two, I need to figure out which herbs are necessary for preservation and aging (the second one will probably need a good bit of aging).
Are there any other herbs I might want to consider?

So a friend of mine who's into herbal medicine and the chemistry behind it, says that Yarrow is an antiseptic, much like hops, and will also contribute bitterness, so that might be my answer on number two.

I have read several treads and articles on this and what exactly is gruit? I know it is a mix of herbs but which ones and how much, how is bittering determined? Many questions. ???

Gruits were the traditinal beers in scotland for a long time and apparently it was yarrow, heather and  bog myrtle that were used.

Wormwood is insanely bitter and also helps as a preservative.  I'm doing my 2nd gruit next week and will follow a barleywine grain profile.  Bog myrtle, yarrow, and heather tips, White Labs 001 Calif ale yeast.  I won't worry too much about preservatives beyond good sanitation.  A little souring just adds to the authenticity, right?