If one live-installs IRIX 6.5.21 on an Octane without applying patch 5086, it will kill the PROM, rendering you unable to boot the Octane anymore! Check out this forum thread.
Be absolutely sure that you have patch 5086 installed on your Octane, if you want to upgrade to 6.5.21. The patch is previously explained in this forum thread.
A miniroot 6.5.21 install should be fine, provided you boot the miniroot from the 6.5.21 install CD.