I've had almost this exact configuration in the past and with the first Windows 8 developer preview that came out in 2011. I even had Ubuntu over the top of everything, I ended up trusting Grub boot manager to ensure everything was fine. But it all worked fine at the step before Ubuntu was installed.
- A shared partition should work fine.
- Install Windows 7 first, and configure Windows 8 VHD next (I suggest this approach because it has worked for me, and for Scott Hanselman in this guide.
- No. Have an "on the tin" Windows 7. Remember the VHDs you have can be backed up fine and re-added to the same laptop without worry if you have to re-pave the base Windows 7 install. Unless you really need to access all the files on the host OS from with the VHDs.
- Size allocations if you go the 'fixed' size allocation of the VHDs, I had to spend some effort it re-sizing the VHDs when it got to the point where the 40 or 50Gb I had allocated got all used up.
Also your 100+ gig allocations sound reasonable. I did all this on the 500Gb HD and struggled a little. Host OS, 2 VHDs, Ubuntu, (no shared partition).