"If you had seen him as a child, you would agree that he is extraordinary."
In this sentence, changing the past perfect to simple past strikes me as wrong, and changing the "would agree" to "would have agreed" also strikes me as wrong. But what I'm left with is something that looks like a hybrid conditional. Basically, the first part of the sentence talks about then, and the second talks about now.
I would like to know whether this sentence is grammatical to begin with, and if it is, whether conditional sentences must always be expressed as on of the three types of conditionals.