It's not a hard and fast rule. Sometimes a triple Hacker will break stealth, and sometimes not. If a triple Hacker is on a pad with a Spec Op, stealth will hold until you put a third unit down, then it's a crap shoot.
There's a limit on how many units you can place with the Spec Ops (I'm sure the Stalker is better). Generally one Grill and one MG unit, or two MGs or three other snipers per Spec Op.
If you can get two Spec Ops on a five pad, then you can pretty much put anything else there.
Keep in mind some infected units will spot stealth units no matter what. Runners most commonly and Doomsdays occasionally.
Stealthed pads dominate in PvP, but are not really necessary in most other raids. Dubai and Bogota being the most useful deployments.
And thanks TBone! Good to hear from you again!
MrMagoo, what guild are you in? I'll be jumping in a week or so and can drop by so you can give it whirl if you like.