Combat Drugs are rolled prior to deployment, so you don't have to choose which squad you are putting your Succubus with untill after you roll.
If you roll a 6, you may decide to put the Succubus, with a different unit to transfer some feel no pain to say a unit of Inubi, then leave them and join your wyches (Basically using your Succubus as a Haemy). Or you can choose to put the Succubus together with the wyches and share the pain to 2 pain tokens.
The worst sort of protection is confidence. The best defense is suspicion.