Some more general advice as you prepare/proceed to run your game:
*Don't try to be a master of the rules from session 1. If somebody wants to play a mage, tell them you expect them to learn the rules for magic. If you've got a decker, tell them you'll be relying on them to help you run decking scenes and knowing the decking rules. Eventually you should learn how everything works, but don't hesitate to lean on the players a little at first. But be open with you intention to do so, and be open with the fact that the first few sessions might be a little slow while you all get a grip on the system (and occasionally stop to check the book).
But also don't be afraid to make a rules call to keep things moving, tell your players you're doing so, and you'll double check the rule after the session, so you know for next time. The books are not always easy to navigate in a hurry, so this will keep things moving.
*Lots of GMs ban decker PCs from the game because decking becomes a solo minigame for one player and everybody else sits twiddling their thumbs. It's not untrue, but I think this notion had more merit back when decking was actually a dungeon crawl minigame in the system. It's a lot better in 5e, and I recommend letting a decker in the game at least once before you decide if they're more of a burden to your game.
*I'm going to offer a counter-point to what the others have said about filling in holes in the team with NPCs. Not because I think that's badwrongfun, but because I think not doing so is legitimate, too.
Shadowrunning teams don't need to fill every archetype. Got no mages? No problem. The way I see it, when Mr Johnson needs some runners to steal a spell formula from Wuxing's Geomancy R&D, he's going to call Mr Fixer and say "I need a team for a job, make sure there's a mage, it's important for this job." Mr Fixer isn't then going to call up the team of runners with no mage. It doesn't make sense for runners to be hired for jobs they don't have the skills to do. It doesn't make sense for the Fixer to contact them for those jobs and it doesn't make sense for Mr Johnson not to be specific if he needs someone with specific skills. It makes the runners look bad, it makes the fixer looks bad, it makes Johnson's life hard.
But more than that, even if they do get a job that thet'd be easier with a certain skill set - and there's plenty of reasons it might happen - Shadowrun is a game where a team of two deckers and three faces can plan a run around their skills and succeed in spite of their weakness. No street samurai? Then they will (or should) plan to avoid fights and plan to get out quick if the fighting starts. Team of four samurai and a rigger? Lots of recon, tactical precision, go in hard and fast, get out faster. Over time, with successful runs, they'll begin to round out the skills that compliment their preferred method of attack.
Sometimes the players might want to round out their skill set with some hired help, but that's a cut into profits, so it probably won't be their go to plan. It doesn't need to be. More than a "well balanced" team, Shadowrun needs players to play smart and to their strengths.
So don't feel compelled to throw in an NPC or GMPC that fills a gap in their specialties. You know, unless that NPC is going to lead to some interesting plot and drama. Then go nuts.
*This is my opinion and it depends on the group you play with and the tone of your game. I lean heavily towards black trench coat games, gritty and sleazy and grimy, with frequent reminders that the characters are living in a dystopia. I also have players who love to role play, and develop their characters, and are self-motivated with their goals. Under circumstances like those, the best sessions are often the sessions between runs, when the characters live their lives, pursue their goals, and try to make things better for themselves and the people they care about.
Some games will go from one run to the next with little connecting them and little breathing space between them. If that's what the group wants, again, it's not badwrongfun, but I think it misses out on a big part of what makes Shadowrun so appealing. At least to me.
*Everything Sphinx said is great. I want to repeat the point about NPCs being ordinary people, even the opposition.
Your average security guard won't have the same skill or gear as your average street samurai. Even many above average police officers won't have that level of skill or equipment.
Shadowrun is a lethal game and shadowrunning is a lethal profession. The ones who make it have to be great at their thing and tough as nails in order to survive. Every now and then the PCs should run into somebody who can put the pressure on them, make them sweat, even kill them if they're not careful (as oppose to killing them with a lucky shot, which anybody can do). But it won't be every knight errant employee who can do that.
But what the opposition lacks in highly trained and slightly mad specialists, they make up for with resources. The law is only a PANICBUTTON(tm) away, and there's a lot of the law. If they really want to, the big corps can hunt you down for your crime, and nobody wants to be looking over their shoulder for red samurai the rest of their life. The sheer size of the organisations runners can go up against, and the consequences for a run gone wrong even after the get away should be reason enough for runners to be careful, even if they've got so much chrome they can shrug off an Ares Predator fired at point blank range.
*Whatever your game ends up being, just have all of the fun with it. Shadowrun is great. And let us know how it went.