Clearly, I do not know much about reducer/flatteners
If the seller insisted that this reducer is suitable, then it probably is. Also the link provided by Tony suggests that this reducer can be used with various FLs. It looks like 2mm extra should get you there.
Having said that, I remember a conversation with a telescope manufacturer who explained to me that quality flatteners are designed for a specific FL. It might not be applicable to reducer/flatteners as much, or at all :-)