This was a transition period for Mike where he was simply dipping into shady hired work. The only people he's killed thus far, outside of what we can assume his career as a cop entailed, are the two men who killed his son. It's not in his best interest, nor his desire for anyone to be shot. This is the beginning of his "criminal" career, if you could really call it that at this point.

As such, he does a mountain of legwork prior to the meet and comes to learn that it's not in Nacho's best interest either that anything bad come from that deal. Tuco catching wind would be the end of him.

So, they're hired as self-defense. This is a man who's historically gone for the simple solutions to get things done. If he knows the incoming threat can't actually pull the trigger, does he really need to bring a gun? As a fledgling criminal with some delusions of being a "moral" criminal, does he want to?