Hi Brent,
The idea of going from a 8" to a 10" with accomodation for the increase in size be allowed for sounds good, but I'd advice some caution. You've mentioned some points already to be weary of, but the secondary mirror is something to not forget about. The secondary for an 8" f/6 mirror is small compared to one for a 10" f/5. You would either need to purchase two secondaries, or an oversize one for the 8" to accomodate the 10" in the future. I too would suggest just to make the 10" from the start.
I'd also suggest determining the design of the scope before determining wieghts and geometery blindly. Work out what design is easiest for you to make, use and store. Sounds simple, and it is, but reasearch is needed.
I've included a couple of pics of the 10"/5 scope I made. It uses only two poles and had a pedigree of some 20 years before I built mine 14 years ago. I've seen this design used in 4" through to 17.5" scopes. I made mine as simple in components as I could, but it works much more smoothly than most commercially made dobs,
. Other than the optics and the focuser, everything else is made by me. Takes three minutes to set up and start playing.
This link shows a much more 'hotted up' two pole design for comparison, and to show how the design lends itself to hybridising, but it is just a compact as mine:
http://www.homebuiltastronomy.com/twotube/index.htm
Here is another ATM'ers site. It goes into some depth in component design and commercially available components. I used this site extensively for inspiration when designing my 17.5" scope:
http://stellafane.org/misc/links.html
Have a good look through the DIY forum. There are many finished scopes you can use to help decide on a design for. My 17.5" is there. An advantage of a 10" mirror is that it is much more forgiving in requirements than a larger mirror.
I hope I've helped.
Mental.