I know that at this point the FE15 modding/hacking community is basically dead after the no reinforcement patches were released. Regardless, this game has a huge soft spot in my heart. It got me through a miserable summer job as a camp counselor, and I'll always be grateful for that.
1. A lot of the enemy/Amiibo classes work WAY better than expected and a decent number actually look pretty natural. I also tested the Amiibo, and all of their classes look akin to the playable legacy classes in Fates (eg. Great Lord, Vanguard, etc.) where it was like your character is wearing Marth's/Robin's/Lucina's/Ike's outfit.
2. The sprite system for this game is so cool! The game seems to use this head-and-socket system for classes where your character's head is put onto the class body (the "socket") to genericize the process instead of making custom sprites and throwing a generic sprite when you can't pull one. So that means characters who you would never expect to see specific sprites for (ex. Sonya as a Baron, Python as an Arcanist, Deen as a Hero) still generate sprites due to how the head-and-socket system works.
3. Spell lists stay the same for the most part. Some classes suffer some minor compatibility issues (ex. Can't learn White Magic as a Balor), but for the most part spells stays the same.
4. Alm is not compatible with randomization. For some reason, Alm basically loses all playability when he classes into something other than Unknown, Fighter, Hero, and Conqueror. He is unable to break boxes and as a result he can't navigate most dungeons. So, for most runs, you should opt to keep Alm in his normal class progression. It sucks, but luckily Celica doesn't have that problem.
Anyway, here's a link to the tool: Click Here!
You can also find a full writeup with the prereqs and some suggested runs here. (CFW and NTR cheats are required to randomize it with the method I'm using. The specific NTR Cheat is the "All Class Selectable by Class Change" option that will allow you to class people into their appropriate random class)
I still have yet to do a full playthrough to confirm that the game doesn't crash during any story events. I've started a run, and as of now, nothing has crashed, so I think that's a pretty good sign, but I'll update you if anything happens. I imagine that the probability the game itself crashes is minimal, since all the randomized classes exist within the game, but it's certainly possible. Only thing I could imagine happening is that something weird might happen with cutscenes with Alm and Celica. I'll definitely post an update when I make it to that point! Anyway, when I get closer to actually making something to randomize the system, I'll post it.
Hopefully some people will make use of this tool! Either way, really happy to be a part of this community and feel like I'm contributing something.