If you want to use this sort of technique, try pencil. An HB lead is a good all around hardness for retouching. You can use a wooden pencil just to start experimenting, but traditionally, retouchers would use a lead holder, like a mechanical pencil but with leads the thickness of a normal pencil, sharpened to a long point with maybe an inch or inch and a half of lead protruding from the pencil. You can retouch on a light box, and if you find it to be a promising technique, you might look for an Adams Retouching Machine, which has a cradle for holding the negative over a light box, a magnifier, and a vibrating base for smooth pencil work.
If the film has a retouching surface on the base, it's better usually to retouch on the base, because it doesn't require as much precision as retouching on the emulsion side. If it doesn't have a retouching surface on the base, you can still buy retouching fluid for the purpose or you can make it (search on "retouching fluid" and I think a few people have posted recipes).
You can use a blending stump, just as you would in charcoal drawing, to smooth out pencil work on negs. A blending stump is a tight roll of soft paper, cut on an angle, for smoothing out pencil work (or pastels or charcoal in drawing).
Of course, it is easier to work with larger negatives, but if 6x6 is what you have, there's no harm in trying. You can always clean off the pencil.