I would not worry about the pH change, as I overstated a bit in my original post. Leaves will gradually lower pH over time (as would driftwood), but it should not be quick enough to worry about a pH fluctuation. With regular water changes it actually takes a good deal of work to change your tanks parameters from those of your tap water.
There should be no problem with the branches as long as you give them a good boil first. As an added bonus, the bark should fall right off in the boil.