Formal

[sugar and spice]

I think there is some overreaction going on here.

Honestly, I don't think that your alums are going to be annoyed if the guys show up in suits instead of tuxedos. But they might be annoyed if they show up and only three girls from your chapter are there because the rest couldn't get their dates to rent tuxes.

It sounds like you are so into putting on a classy event that you are forgetting what a formal is ultimately about -- having fun with your sisters! While your aim to make it as classy as possible is great (why bother spending tons of cash on a formal if everybody dresses like they did at your last date party?), the way you're handling this will only serve to make your sisters, their dates and especially YOU more stressed out. Worry about yourself and let your sisters take care of themselves and I'm sure the formal will turn out fabulous.