Will I risk breaking Tails doing this? Is there a more recent post someone knows about this I didn't find in my searches? Is this install really as simple as it sounds?
Right, I got this finished, finally, after a corrupted install and a bad USB stick. The only process needed is the one by /u/TailsPerks here /post/a5cee015fc8ce3c9f99c/#c-bb99a312f6327ecb46. Anything beyond those instructioins corrupted the installation for me.
Then if you want to get super anal security, you can get a encrypted keypad USB drive like a Kingston Ironkey USB or an Apricorn. They aren't cheap, but a worthwhile investment in my opinion.
No you will not break Tails doing this. I added programs to Tails without issue. Gajim supports the Tor proxy natively so that won't be an issue.
I do suggest after you are done doing your admin password work, you reboot so there isn't an admin PW set for OpSec/paranoia reasons.