Guardian Angels are very useful in mono Light and Light/Gravity, at least.
Removing poison is way OP... it makes poison-based creature control totally worthless against anything more than */1. Things are hard enough to kill already - with an Angel, */1 takes 1 turn to kill, */2 takes 2, */3 takes 3, etc. AND it completely occupies your Parasite / Blowfish (of which you can usually only have a maximum of 6 at a time) for that many turns, and the enemies are still doing damage for that many turns.
Buff is totally not needed.
The Potential card does seem like a most excellent idea, though. It would make the Angels really quite powerful, and further encourage the underused Freeze + Shockwave combo.