Just don't use drugs twice, or they die.
Actually they die when their non-juice-adjusted health drops to zero, and it does a point of damage whenever a random roll from 0 to 49 (I'm pretty sure those are the bounds; it's LCSrandom(50)) is less than the number of drug sessions (starting at 1) they've endured - also whenever they fail an attribute check against health proportional to the power of the interrogation in a beating. Since their health is displayed with juice-adjustment, it can appear like they die randomly. CCS members for me always seemed to die at 4 health remaining.
As for costumes, there are three factors: base power, assault bonus, drug bonus. Assault and drug bonuses do not apply if you are not assaulting or drugging them respectively.
Here's the list of armours that have any bonuses (name(s): base, assault, drug):
Ninja Suit: 0, 0, 4
Trenchcoat, Security Uniform, Black Robe, Body Armour, Heavy Body Armour: 2, 0, 0
Clown Suit, Elephant Suit, Donkey Suit: 0, 4, 4
Cheap Suit/Dress, Tactical SWAT Armour: 4, 0, 0
Black Catsuit - 4, 2, 0
Black Suit: 4, 4, 0
Expensive Suit/Dress, Lab Coat: 6, 0, 0
Masks: 4, 4, 4 - all masks?
Police Uniform, Black Dress, Army Uniform, Guard Uniform, Police Body Armour, Army Body Armour: 6, 4, 0
Bondage Gear: 6, 4, 4
Death Squad Uniform: 8, 6, 0
One notable point of confusion: as far as I can tell, the assault bonus is totally useless. It's never referenced in daily/interrogation.cpp. Also, only the costume of the lead interrogator counts for the other two bonuses (though that would be as intended I assume). Therefore the only two costumes you need are the Bondage Gear and the Death Squad Uniform; the former if you're using drugs and the latter otherwise. If you can't get a Death Squad Uniform then the Bondage Gear is the best in every aspect.