>> Yes, but using a master-only panel is not mandatory.
By default every new install will set the panel to stand-alone master server.
You can add it as slave using this guide :
>> If you want to have a separate backup server then you can add it from Servers->Add backup servers.
And use that backup server in vps backup and also for database backups
The extra space plan:
>> Vps can use space from storage added on panel.
If there is space is same type of storage then you can increase primary disk space.
You can even have multiple disks for vps from multiple storage on panel.
But using a storage space from another slave is not possible, unless its a shared storage and it is available across servers and also present on panel as storage.