Hey Nomad,
From my experience, you always let the players make characters, its such a compelling part of this system. But yeah, restrict the number of playbooks, really just dropping some of the complex ones which may not normally become effective or most interesting until later in the campaign.
Have a strong setting in mind, and something immediately threatening to the characters. Don't let them wonder what to do in those first moments, there must be something threatening them, their community,their loved ones. Something un-ignorable. It can be overcome (or not) quickly, but get the characters struggling against something from the start, forcing them to choose sides, work together, risk their lives, take risks.
I've done a couple of con scenarios, would be happy to talk to you about them.
Scott (Yes, that Scott)