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The general rule is the longer the better and more of them for improved signal to noise ratio. However if your tracking doesn't allow for that then you can stack lots of shorter exposures, but you won't get as deep.
The internal noise reduction is ok to use but there are disadvantages. One is that they take up valuable time that could be used for imaging. As long as you don't get variations in ambient temperature you can always take darks in 'blocks" during the night or even at the end when you are packing up. A good routine would be take 10 lights and then 5 darks, 10 lights 5 darks, for shorter exposures. For longer exposures you could try 5 and 2, 5 and 3. That way no matter what happens you will have lights and darks to work with.
As for ISO 400 to 800 seems to be fairly standard.
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