Good effort on the mill mate! You can mill almost anything you like, as it is always more valuable than firewood, although you may not see a top return for a year or two for drying. Even though the trees you mention have been dead for a while, wood in the round just doesnt seaon. My fave tale is when my mate bought 30 acres of woods next to his farm, and we couldnt understand why this pisece of hedge had been built- 12'long in the middle of the wood. Until i noticed what appeared to be a tall tree stump a few feet off. I tturned out that the 'hedge' was an oak log, the sapwood had rotted completeley and there was about 6" of earth (years of rotten leaf matter etc) with a healthy selectin of understory growing off it! We dragged it out, cleaned the crap off and milled it, and it was sodden in the middle- as wet as if it had just been felled.
When you have milled the timber, it is essential to dry it out properly. This requires a level base, ideally concrete, with some sturdy (3x3?) bearers every 18". Put your first board down, then directly abovr the bearers put some 1" stickers, then your second board etc etc all the way up. Ideally you want to weigh the top board down as much as you can. Another milled log is pretty good, and banding the boards "in stick" with the sort of bands you get round concrete blocks etc. (although personally i dont bother) The wood doesnt have to be kept indoors, some say it is better not for a year or so, but they dont want to be in direct exposure to the elements either. A pole barn is ideal. Dont cover them with plastic. To air dry in this country, which will get you to a minimum of approx 20% moicture, the rule of thumb for hardwoods is 'a year per inch, and one for luck'. i.e a 2" board wil take 3 years. Softwoods half that. For flooring or funiture use you will need to get it in a kiln to reach 10-12% moisture. If you are making garden furniture then you can make it green.
As for thicnesses, it is very difficult to say. For commercial milling, all thicnesses have their place. For yourself just think of the final use for the timber- kitchen sideboards, window ledges etc. Oak is a good seller in lintel sizes if you find you have loads and want to shift some.
Hope some of that helps. Happy milling.