A zpool is also a zfs filesystem, so you can, yes, write files to it. You can also create snapshots of it, etc. I'd generally recommend not doing so simply because you have more flexibility if you create a zfs filesystem inside it -- you can then create others later, assign quotas, etc etc.
Nothing will break, though.
And yes, if you have hardware RAID already you don't need to do software RAID inside ZFS. RAIDZ is more efficient than most previous software RAID5 etc., but it's still (a) slower than hardware RAID most of the time, and (b) unnecessary to duplicate.