First of all this setup is for a proof of concept. The existing disk is ext4 formatted. A total of 4 instance groups created to troubleshoot. The status of 3/4 instance groups has been "Group is Resizing," some for over 4 hours. A minimum-2 and max-5 instances set between all groups for consistency. Only stateless Instance group 3 works, see below for further group details.
Edit: Each instance group is based on an instance template
Stateless Instance group 1:
f1-micro -> Ubuntu 18.04 -> 10GB Boot -> with Startup script -> Attached Existing Disk (20GB) SSD Persistent Disk
Stateless Instance group 2:
f1-micro -> Ubuntu 18.04 -> 10GB Boot -> noStartup script -> Attached Existing Disk (20GB) SSD Persistent Disk
Stateless Instance group 3: Only one that works
f1-micro -> Ubuntu 18.04 -> 10GB Boot -> noStartup script -> No Additional Disks
Stateful Instance group 1:
f1-micro -> Ubuntu 18.04 -> 10GB Boot -> noStartup script -> (Stateful) Existing Disk (20GB) SSD Persistent Disk