Luckily you resist Compulsion with Willpower + Charisma. Since it's a power, you can't counterspell it.
Compulsion's description does suggest that it should be for a specific type of action depending on the species of paracritter. So in the case of it being an optional power with spirits that you choose, i'll say that you need to specify this when you take Compulsion. Since I didn't know this before and Zach already knows what he wants to compel and this is the first use of Compulsion we'll just say that the action which this spirit compels is to have friends attack friends. So from here on out you can only use it for that and can't compel other types of actions.
So if Brick fails the resist roll then he will be compelled to shoot Thermo on his next action phase only.