Let's just cover Win 7 partition size and Steam for now.
Here's what to do with Steam.
This is how I did it. https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129
Put your games on a different drive from your system drive.
Do that before you touch your system drive.
Consider your games to be data and back up data separately from your system.
Here's what I suggest.
Make sure you've backed up needed data to your non-system drive.
Partition your system drive with a 100gb partition for Windows 7.
Then install Windows 7. Let's call this the C: partition.
Can't say what to do with the Win 8. I'd delete that partition myself, as I think having 2 OS's is a needless complication.
Install the apps you normally use to their default location in the Win 7 program directories. Tweak your desktop as you like it.
Immediately use Macrium to create an image of the C: partition.
You can find how to do that on this forum. You will burn a CD using the Macrium WinPE ISO download whs has provided. It works.
Put an image to 2 places. The non-system drive and for now the other partition on your system drive. Think about getting an external drive for the second copy in the future.
The images won't be big, about 15-30gb depending on what you install for your normal working system.
We can bat this around, but that's a basic start for you.