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The rules in the Chaos Codex for the Thrall Wizards state the following;
"When the Sorcerer wishes to use a further power a Thrall is Sacrificed and the psychic power is cast automatically"

Codex Space Marines, P24, 'Psychic Hood'
"Declare that you'll use the Psychic Hood after an opponent has successfully made a Psychic Test, but before they have used the power."

Codex Chaos Space Marines, P59, 'The Mark of Tzeentch', "Independent Characters, Chosen, and Posessed Space Marines."
"All models automatically have the Sorcer ability without having to select the upgrade from there list category. In addittion they will automatically pass any Psychic Test they are requred to take."

So Psychic Hoods only affect powers that take a Psychic test, and while the Mark of Tzeentch allows models to pass the test automatically they are then subject to the psychic hood. Thrall Wizards however cast the power automatically, they dont pass the test automatically, they simply cast the power automatically.

Am I right in my interpretation?

The problem is, what exactly does casting a power automatically mean? I personally would say that they automatically pass their psychic test, but I guess an equaly valid conclusion would be that they automatically cast the power without having to pass a Ld test.

I'd certainly call this a grey area which means you should stick with the less advantageous interpretation.

I thought it a power does not require a psychic test a Librarians Hood cannot dispell it. It states in the Nid codex that because an unfocused warp blast requires no test it cannot be dispelled. Also it says in the Hood description that the power is able to be cancelled after its psychic test has been passed.

But Tzeentch sorcerors don't take no tests, they just automatically pass them. That's not the same as a Warlock or some Nid powers.

My interpretation:

The hood dispels powers after the guy passes the psychic test. You automatically pass them, so after you would take the test, he can dispel it.

As for the Thralls i think that they are dispelled after it is declared cast.

I think GW just wanted the thralls to enable sorcerers the ability to cast extra powers and since you pay for them you wouldnt have to take a test (you might just waste the points after failing).