Breaking a volume into subvolumes is one of the most useful features of a filesystem like btrfs. You should definitely do it. The more the better in my opinion.
You can create subvolumes whenever you want. You will have to move the data and setup mounts for them in /etc/fstab.
IMO, this totally defeats the purpose of using a filesystem like btrfs in the first place. One of the big advantages of using a fs like btrfs is that you can separate volumes with there own policies and snapshots without having to separate the space on the physical disk or worry about which is filling up first.
The counterpoint to this is that if your /home is on a separate partition it makes it easier to install a different distro without erasing your /home partition. I am not really a fan of that practice to begin with though.