According to David Lebovitz:
Because natural cocoa powder hasn’t had its acidity tempered, it’s generally paired with baking soda (which is alkali) in recipes. Dutch-process cocoa is frequently used in recipes with baking powder, as it doesn’t react to baking soda like natural cocoa does.
So, if you're using non-Dutched (natural) cocoa, you can use baking soda for leavening, but you don't have to. As I mentioned in comments the vast majority of recipes for chocolate biscotti in fact call for baking powder. I suspect that part of the reason for that is that many people don't know the difference between Dutched and non-Dutched cocoas, and in some places outside the US, Dutch-processed is the norm. There is no harm in using additional acid (like natural cocoa) with baking powder. The effect of additional acid is not enough that you're likely to even notice it.
Also note that if you use baking soda for leavening, you have to bake right away. Baking soda, like single-action baking powder, causes the release of carbon dioxide only when it is first mixed with the liquid (and acid, in the case of soda) in the recipe. It doesn't react to the heat of the oven. So, its leavening action is short lived.
Baking powder is roughly one-third baking soda, so if you do choose to substitute baking soda for baking powder in a recipe that contains plenty of acid, you want to use about a third of the amount of soda as the recipe called for baking powder.
EDIT: I just came across this answer: How can I identify dutch process cocoa? by hobodave. That excellent answer has more information that you might find helpful. Of note as it relates to your question is this paragraph:
There is a bit of misinformation that floats amongst bakers that the pH of the cocoa can affect the leavening of the baked good. Many recipes will actually sternly suggest using either Dutched or natural cocoa depending. This makes sense since leavening is a sort of balancing act that involves both acids and bases. However, it has been experimentally shown that this does not actually occur, and baked goods made with both Dutched and natural cocoa powder showed no differences in leavening.
So he concurs. While it is fine to substitute baking soda for baking powder when introducing natural cocoa, it is unnecessary. No modification of leavening agent is necessary.