How can I get "Automatically fill free space with songs" to reserve more space than a half gig
Unfortunately no, the automatic filling takes place during sync, and only gets changed in size if you add/remove things that will also take place during sync.

If you want to limit it while also preserving some of that randomness, try creating a Smart Playlist that will select songs at random, and then tweak the "Limit to" feature until you have enough space free. It limits to number of tracks not size, so it won't be exact, but it will allow you to (roughly) tailor your size.

Edit: Actually I made a mistake, it is possible to limit to a GB amount


Create a Smart Playlist
Under "Match the following rule:", select something broad like "Media Kind", "is", "Music"
Tick the checkbox next to "Limit to"
Select "items" and change it to "GB" and enter your desired GB amount