I really like the idea of the GM in black shirt, black vest and red tie. The GM in Yoko & Shane's wedding look awesome, really sharp and suave. I like it much better than having the black shirt, red vest and red tie... as that kinda
feels a little to valet-like to me.
For your FI, I think ivory shirt, vest, and tie will look really nice too. It will probably look a bit more formal than his GM, and he'll also stand out really well as the "groom.". To keep a tuxedo theme going, your FI could wear
the same as his GM, but with an ivory tie. I think that would look sharp too.
My own preference: I'd put the GM in black shirt, black vest, and red tie. And put your FI in ivory, ivory and ivory. I think it'll just look nicer in photos. When your FI is with his boys, he'll stand out. When the photos are just with
you, it will balance better... instead of being a stark black silhouette with a stark ivory silhouette. Does that even make sense? (it does in my head!)
Good luck!