Does installing GApps save space compared to PlayStore install?
Question 1: Is there a drawback to making the /system partition as full as possible by selecting the biggest fitting GApps package?


Whether you install the smallest or largest version of GApps, /system partition will remain the same, unaffected in term of its size. The partition size is fixed for the device by the manufacturer (Yes, if you are rooted it is possible to re-size the partition but it is not recommended).


  Question 2: If I need some Google apps such as Maps and YouTube, which option maximizes the space left available to install other apps, updates and user data?


When you install GApps from the recovery, they will be installed as system apps under /system partition. Then when you start your device, their data will be under /data partition as any other apps. Subsequent update and usage may (will) increase their sizes on the /data partition while /system will be same. Making new version or updates part of /system partition will allow to recoup some space from the /data partition.

If you need Maps and Youtube, install GApps mini, then disable/uninstall apps that are not part of the pico or nano packages.