I just grabbed the 5th Ed Quick Start rules to take a peak at what you're working with (they can be found
here, btw, for anyone else that might be interested).
After a quick overview, it looks like the presented run is primarily a little bit of negotiations, some legwork, a major fight between the (four) players and a bunch of mob goons (five in total) and then resolution. If you're planning for this just to be a one off session to see if they'd be interested in doing more, I would probably just have one of the characters double up; ideally two Combat Adepts or two Street Samurais with the mage as a close third (two deckers probably is overkill for the run and won't give one a chance to shine during legwork). Its a one off, so there's not a lot of sense in spending a bunch of time statting out a unique fifth character.
To balance out the combat, I would just add an additional mobster, again just duplicating one of the existing stat boxes and giving him a new name. The rules mention that the security guard, "Lucy" is feeling a little nervous because his brother came down sick tonight, so maybe in your game his brother
isn't sick and is hanging out in the back or in the bathroom at the time the runner's enter. That'll give you two security guards in addition to the other mobsters to throw at the team.
In the long run, I wouldn't worry about it being too easy, perse. It is supposed to be an introductory adventure, not an uber challenge of the player's skills. Make sure each player gets a chance to shine; let the decker excel at the legwork and tackling security systems, let the Street Shaman get into a magical battle with the Mob Mage, have the Adept(s) duke it out with some of the mob members and have the Street Sam(s) shoot it out with the guards. As long as everyone has fun, you'll be in a better spot to pitch a full on 5th Ed Shadowrun campaign and be designing new characters from scratch.
