Sorry, slightly more snide than I intended. Take it more like "do some playing/testing with the flash before the event". If you don't have a DSLR, load up some cheap ISO100 negs, set camera ISO to 64 (the extra exposure won't bother the film but will give you an idea of the shutter speeds achievable), shoot a roll indoors and take it to a 1-hour lab.
Try flash in full-auto, direct and bounced. Try slow sync. Try slow sync and about -1 flash compensation.
You want to balance ambient and flash if you can, except where the colours will be horrible. Balancing is much much easier with faster film since it means you can get more ambient in with acceptable shutter speeds. So I'd recommend taking some 400NC or similar for this event.
A faster lens will also help you balance against very low ambient.