Customizing partitions with parted on an installed system should not prevent upgrade from working, so you can try it (use common sense, of course). However, 1GB is not enough if you are planning upgrades, I’d leave at least 4GB for the root. As of one-to-many NAT, if it’s reproducible, please file a bug at bugzilla.vyos.net (include the config and relevant debug information like traffic dumps etc.).