My instinct is that if you're just telling your people to go do something, then they (probably) go try to do it, and succeed or fail at the MC's discretion, same as any other NPC thing. I think using your gang as a weapon for violence moves is the exception here, not the rule. (Per the 2e final preview, "When you have a gang, you can sucker someone, go aggro on them, or make a battle move, using your gang as a weapon." Nothing about any other move.) If you send some scouts, they'll come back and give you a report on what they saw (at which point you can ask your followers for updated advice), but I don't think the rules support you Reading A Sitch through them.
Go Aggro by poisoning, either directly or through a gang, feels a little off to me. I think I'd be inclined to make the part where you get to the water supply the interesting part, if you were doing it yourself, which would probably entail Doing Something Under Fire. Then once you get there, you just dump the poison in and the effects of that play out predictably and fictionally. If you're sending a gang off to do it for you, then that just happens off-screen. But I think an argument can definitely made that you'd be Suckering Someone (with a chance to miss) by doing this, so this one is more of a personal interpretation thing than a rules thing.