hi lft
are you going to have someone helping you ?
if you can, i would have someone hold a white card as a bounce / fill ..
the best thing you can do, when embarking on a street-venture like this
is get a friend and do a set-up / dry run to see how things worked .. ( on the street, of course! )
do a few different ways, flash that bounces on a card and then onto your subject,
low shutter speed drag the flash, just use a reflector / card to work as a fill and no flash &C
take notes on what you did and when you get your film back from the mini-lab recreate the best set-up
for the real thing.
i agree with sam h. don't use a direct flash
have fun!
john