If the question is both abilities hypothetically cancelling each other out, then yeah Immunity would cancel out Toxic Boost since Zangoose would never get poisoned by the Toxic Orb. If it's not hypothetical though, then you can't have more than one ability at a time, therefore you'd either get the boost or have nothing depending on ability. In other words:
Immunity: Zangoose won't get a boost or any poison
Toxic Boost: Zangoose WILL get poisoned, but it'll get an attack boost.
Both: Nothing would happen since Immunity would just prevent Toxic Boost from activating.