You would get your point across, but it is not a good translation as "stuff" is both more colloquial and less specific than "stoffer". If the original had been "Inneholder den farlige ting?", then your translation would have been closer to the original.
I understand that den is for masculine/feminine and det for neuter, but is there any specific reason why den is used here? Or do we simply assume we were talking about something masculine/feminine before?
What would it be for "does this contain dangerous substances", which is what I had.
"Inneholder denne farlige stoffer?"
(But if this is referring to, say, a house, it would be, "Inneholder dette farlige stoffer?")