See what fits - try it out with all girls, then all guys then a mix of the two. I always wanted an all-male band but when it came down to it, we had a second female singer and a girl on keyboards and it worked really really well with the four other guys in the band. What you don't want is a group of egos refusing to back down and trying to be in charge all the time. It has to be a team effort or not at all - it won't work if there's a power struggle going on.
Just do a bit of trial and error - test each dynamic and see who brings what to the group. So long as you can get along 98% of the time with whoever winds up with you, it'll work whether it's all guys/girls or a mix