small cabin advice

[CJennings]

I'm going to be building a small cabin, 10 feet by 15 feet, in northeastern Vermont, more or less based on Thoreau's cabin at Walden, for a shelter at my property until I have more money to burn on my camp. It will be mostly timber framed with 6x6x8 posts, 6x6 plates, probably the same for the rafters. To cover it I figure on using 2x4 studs, rough sawn 1x lumber over that for sheathing and whatever I side it with beyond that. I may also frame the floor more conventionally with 2x8's or 2x10's for joists and build the timber frame on top of that level platform. I will be hewing the timbers with an axe from trees on site, and seeing as how my time may be limited to produce perfectly flat timbers this winter and spring I think the dimensional lumber may be my best option for the floor versus hewing all the joists. The roof will be timber framed with 4 pairs of rafters equally spaced across the 15 feet length (and cooresponding posts under each rafter location on the wall frame) and a ridge pole and purlins between them.

Now for my questions. I have the Timber Framing Book by Stewart Elliott and Eugenie Wallas which I've been using as a guide as I plan this but I seem to have thought up questions they didn't think of. My little woodlot (10 acres roughly) has a diverse composition including balsam fir, red spruce, eastern white pine, and a variety of hardwoods (sugar maple, ash, black cherry, red maple, yellow birch, aspen, white birch and so forth). There is the red spruce and white pine I can work with and I suspect those are my best options for the more important timbers. However I have a lot of mature balsam fir I'm trying to eliminate and I want to get more pine and spruce regeneration. So I want to cut as little of my mature pine and spruce as possible while I work on taking out fir. I will have a lot of balsam fir around I can use if it's suitable. Should I even consider using the balsam fir in the frame? Perhaps for the purlins as those will be a short span, and somne of the posts?

My second question is on braces. I'm looking at 4x4 braces but is there any advantage to a pegged mortise and tenon joint vs. using metallic fasteners (spikes) on flush cut braces on such a small structure? The braces will be concealed when the interior is finished so appearances are not important to me.

Lastly, and maybe I'm over analyzing this, but: my timbers will be green. Should the pegs be dry or green? And will mixing species (i.e., oak pegs in spruce or white pine timbers) cause issues with different expansion and contraction rates leading to loose pegs or split timbers?  ???