There are usually a couple of ways to correct parallelism. Often turning each item into an "ing" form is the easiest. You can also make them a noun or a "to" verb form. However, some situations require re-ordering the information and taking one of the items in the list and putting it before the subject. I think that is the case in this sentence because "considering their boss" is not quite clear. What does it mean that "the dismissed workers were considering their boss?" I think what you mean is that they were thinking about their dismissal and the fact that their boss is the problem and that is the cause of them marching, chanting and singing. Here are a couple of ways to do this sentence more clearly:
1. Thinking about the unfairness of their boss, the dismissed workers marched along the main street chanting slogans and singing songs.
2. Remembering their boss, the dismissed workers were marching along the main street, chanting slogans and singing songs.
3. The dismissed workers were marching along the main street, chanting slogans, and singing sarcastic songs about their boss.