How can we say that tzadikkim are perfectly good if we see so many historical tzaddikkim who struggled in different ways (i.e. Moses, Kings David and Solomon)?

Answer:

The struggles of the righteous (Tzaddikim) are not between good and evil - rather between good or even better. So for an ordinary person, striking a rock instead of talking to it would not be considered a sin. But for someone of Moses' spiritual stature to strike the rock was considered not good enough.

In other words, tzaddikim, despite being totally good, struggle as well - but their struggles are on a much higher spiritual level than ours.