I’ve had this happen more than a few times and I feel horrible when it does! Really really horrible! Unless its for solo photos of us all.
I usually ask if they are sure as we are a group, then I ask my group. It hasn’t been a problem to others so far, but I sure feel like poo, like I am being disloyal, but also aware that the person wanting the photos may be a fan of the character for really personal reasons and I don’t want to disappoint them.
So it means when I am in a group of similar costumes but not really a part I will usually drift away to the side when there is more interest in the others. If told to get in the photo I will if there is enough unity but otherwise will explain that I am form a different series or timeframe to the rest.
That way I feel even if I can’t make up to my group in that moment they are aware that I am not a camera hog and only interested in my own photos. I like everyone to have their time to shine.