Regarding the disguise, concepts, don't forget that if someone fails a roll, they can try again a few minutes later if they have another opportunity to do so. In other words, if you have people spending some amount of time surveilling a building in advance while disguised, your guards could have the chance to see through that disguise more often than once.
Also, make them roll Etiquette while disguised to make sure they're acting appropriately. In my game, the team had to get into a prison and get someone out. One impersonated a guard and before they did so, they captured a guard, interrogated him for a few hours to learn about his day, his duties, his friends, his attitudes toward other guards, etc. Then, I called for Etiquette tests with reasonable frequency (usually when he met "new" people). Most of the guards got a chance to see through the disguise, then if he failed the Etiquette tests, they'd get suspicious for a moment, and get that handy +3 for focusing on a Perception test. Fortunately for him, he was passing the Etiquette tests, so it never came to that.
On the other hand, if they're just disguising themselves to avoid detection (which is also smart), then this sort of scenario doesn't apply as much.
If you're worried about tossing in a mage, then toss in an Adept or something magical that might ruin their day. Give the adept Great Leap, Wall Running, and a super-high Gymnastics skill and have him running at them on full-defense until he closes and then go ape-shit on them in close combat. Hell, have a friendly mage use the Deflection spell (in Street Magic) so he gets even more dice for not being shot. Finally, remember that the mage need not be a combat monster to ruin someone's day. The worst mage I think I ever tossed at a team had a spell load out something like this (working from memory, it was last year or so):
- Double Image -- Make copies of the shooters so the team has a 50% chance of shooting the illusion and not the actual person.
- Agony -- Hurt them without lasting damage. It's the nice way to take down a PC without forcing them back to character creation.
- Confusion -- start giving them extra negative modifiers
- Sight Removal -- same as Confusion but can compound the effective use of tactics to impair visibility in the first place.
- Phantasm -- make illusionary guards for them to fight; especially effective if they come around a corner *behind* the team!
- Deflection -- bullets miss people on whom this is sustained
- Blast -- indirect area combat spell just for some offensive capabilities
- Stunbolt -- direct combat spell, again, just for some kick
This is more a utility mage sort of thing and less a combat rocker. Toss on Swarm or some other mass illusions, too, and have the mage specialize in illusions (for the extra dice) and if you can safely cast force 6 or so spells with the character, you can make illusions that the average character will not see through. 'Course, if they're using technology to enhance their vision (e.g., glasses or goggles with visual enhancements) that they haven't paid for in essence, you'll need the physical versions of those spells and you'll need to be the object resistance of their gear (probably 3), but if you're shooting for 4+ hits anyway, then you should be good to go.
Don't forget to Fetish the high drain spells for the extra 2 dice to resist the damage and toss on a Focused Concentration to help as well. Hell, if it's a corporate facility or arcology, the mage could all but live on site and get the Home Ground quality, too. All told, you now have a mage that's going to seriously ruin the PC's day but not necessarily turn them into a pink mist.