Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Once the wood is covered with plants there would be only the little bits showing. Even if it is a bit different I do not think anyone will notice. Especially if all the wood gets dark.
As for the different shapes, stout stump-like chunks vs thinner, branching material, yes, that could be a problem, but again, when the plants cover it who is to see that?
If your goal did not include so many plants growing on the wood I would not mix woods. Even the grain of the wood is picturesque and would look funny if there were several kinds visible in the tank.