Well, here's my opinion on the tux/suit thing.
My SAI chapter and our local Phi Mu Alpha chapter have a joint spring formal. The neat thing for the Sinfonians is, because they're in ensembles like Wind Ensemble or Concert Choir where guys HAVE to buy a tux for concerts, they already have their formal wear!
BUT...for me and my sisters dates, we left it up to them. In fact I TOLD my date not to rent/buy a tux unless he just wanted to, because that's a lot of money to shell out. It'd be different if say he asked ME to HIS formal...but I asked him to mine, so I just wouldn't have felt right telling him to get a tux. He wore a very nice suit, and so did many other girls' dates. I didn't feel it brought down the classiness of it at all. We all had a blast.
I think the most important part of a formal is that all the sisters can simply BE there, and their dates should wear the best THEY have or can afford. I think a nice suit is perfectly acceptable.