Dan,
I 'm glad that you have decided to use timber harvested from your property. Keep in mind thought that you should have the timber graded. Strength characteristics vary greatly with slope of grain, growth density, and knot size. Using the wrong stick in the wrong place could be problematic.
As to mixing woods of different moisture content, species, and age; I say go for it. We have been doing this for years now in our frames with no ill effect.
Timber framing is a very forgiving craft and wood a forgiving medium. Look in at some of the old barns around you. You will see woefully undersized sticks showing heavy deflection that have not failed. Admittedly, you probably won't want to see the same behavior in your home, but it does demonstrate the flexibilty at hand.
My pricipal caution remains that you should have your timber graded by a licensed professional as well as have you design evaluated by the same. Having your timber frame drawings stamped by an engineer can go a long way to easing a relationaship with your building inspector. And even if your area doesn't require one, it will make you more confident in your joinery choices.
As a final caveat, stay away from dovetails. In most situations, the combination of wood shrinkage and short grain sections produce a very weak joint.
Good luck and enjoy,
Bob Smith
