I just cooked some apple bread, and I'm curious about adapting it to other fruits. I'm led to believe that the acidity of a fruit will cancel out some of the effect of baking powder or baking soda by neutralizing it, so more acidic fruits will require more baking soda to compensate. Is this accurate? If so, where can I find a comprehensive list of PHs of various ingredients, and how should I calculate how much extra baking soda to add?
It would be nearly impossible to gauge the effect of a different fruit on a quick bread without knowing the types and amounts of other acidic things in the recipe. Milk or buttermilk are also acidic, as are some other things you're likely to find in a quick bread batter.
Your recipe is going to be calculated to have enough leavening action for the bread, using whatever acids are initially required. Most likely this is partly the fruit and partly things like milk or buttermilk (both of which are acidic, buttermilk being more so).
Using more acidic ingredients than intended will just leave your final bread slightly more acid--probably not enough to bother you, particularly if the fruit was to your liking to start with. You may find that a more acidic batter causes the baking soda to rise faster than you want, so you should watch for that. You will not be able to make the baking soda release too much carbon dioxide by acidifying more--it'll do as much as it can then stop.
If you use less-acidic ingredients, you risk making the baking soda underperform. What I would resist doing in that instance is adding more baking soda, however. You won't increase the leavening action and you may find you introduce an unpleasant metallic taste. Better in those cases to let the batter sit just a little longer to give it more time to leaven, or add more of another acidic ingredient.
Ultimately, I'd be more concerned about the varying moisture levels in the fruits used than the acidity. Apples are relatively dry compared to, say, strawberries or blueberries, so you may find that your batter is too wet to cook properly at the time and temperature prescribed.