I so wish I had a suggestion for you. I have gained 125lbs. on Seroquel. I think the stuff is poison and I am so sorry I ever started taking it. I am working on going off of it, but of course, that's a slow process.
No doctor has ever suggested Metformin to me. I really don't know why, since I've gained so much.
All I can offer is that my older sister was on Metformin and had very difficult side-effects from it, including hair loss. But she is not on any psych meds and does not have a weight problem. She was just showing some signs of pre-diabetes, so her GP prescribed Metformin. My sister no longer takes it.