Do you have a list of how many bosses are fightable?
Guard Scorpion, Air Buster, Aps, Reno, Sample:H0512, Hundred Gunner, Heli Gunner, Rufus + Dark Nation, Motor Ball, Midgar Zolom, Bottomswell, Jenova:BIRTH, Dyne, Reno + Rude, Gi Nattak, Materia Keeper, Palmer, Red Dragon, Demon's Gate, Jenova:LIFE, Schizo, Jenova:DEATH, Corel Train, CMD Grand Horn, Carry Armor, Rude, Diamond WEAPON, Elena + Reno + Rude, Proud Clod, Lifeform Hojo N, Jenova:SYNTHESIS, Sephiroth.
Have you thought about moving them to previously empty or dummied out scenes so you could have this as an optional add-on to the game instead of a conversion? In theory you should be able to copy entire scenes and just adjust the battle called for that.
Well I kinda started modifying some bosses and it eventually evolved into what it became. Had I from the start decided to make ths a mod, I might've done that. Although I'm not sure if there would've been enough free spaces...
How long did it take you to create the new attacks, AI and(?) animations(if any)?
No new animations, just a lot of ProudClod'ing. As for AI, I always tried to at least keep the spirit of the original battle a little bit. Take Guard Scorpion for example. He still uses his Search Scope, but for one, it reduces your HP to 1, secondly, he casts Bolt3 on himself during his Tail Phase, and third, his Rifle inflicts Berserk which might cause a character to trigger his counter. Aps was a lot simpler, I just adjusted his abilities a bit to make them more unique and had him absorb Water. From that point it was just determining when he should use what.
Some bosses had an entirely redone AI and attacks, like Rufus. A lot of the multi-enemy battles are similar in nature though, there's one that takes care of the other while one is the big hitter.
I spent a lot more time planning than coding. All in all, I'd say one boss on average took around 5 hours' time.