Adding Zero in X2 is feasible and I actually accomplished it overall on my old project. Just never got around to fine tuning and finalizing a lot of little details, but it's doable. It's one of the more annoying things to do since the X series handles it's data a bit different than other games. It takes a lot of patience and messing around with the right code and finding empty space, etc. It's honestly just a pain in the butt.
Big fan of your work Justin. It's what got me thinking of taking on X2 and X1. Actually commented on one of your X2 vids recently on Youtgube because it got my creative interest flowing. Jacob Hoffman.
X1 is an oddity. I've seen a couple hacks with Zero in it but as his character doesn't have many options built into him so far I'm guessing itd probably be even more difficult to put him in there.
Though the gentleman suggesting using the X3 engine even with sound irregularities makes me wonder if that may be the best way to go to approach this.
Ultimately the sort of hacks I'd ultimately like to do are on the PS1 but from preliminary research it seems quite difficult to do. There is no where near as much support in rom hacking for PS1.
If I do I'll probably monkey with the PC variants since my main gripes are in sound files which are easy enough to fix. I've overdubbed a couple of the .avi vids and ran them with no problems.
In the Korean X6 one day I'd like to add a black zero capsule and tweak the delays between attacks on zeros sprite but that's about it.