Thanks for the response Roger. Do you think the blue shirt would be preferable to the lavender shirt? My biggest concern about the lavender shirts is that they'd be too close to the bridesmaids dresses, but they won't match it exactly and I think it's safer to go for something different.
I'd prefer to keep the waistcoats matching the suits, although the back of my waistcoat was going to be purple to match the lining, whereas the groomsmen would have grey like the suit.
I'll definitely look into black oxfords, they'll probably be more interesting than derbys. Thank you again!