As cool as those anecdotes sound, I am not sure that either of them are true.

Homer Plessy

The Wikipedia article on Plessy v. Ferguson explains the following:

“In 1890, the State of Louisiana passed the Separate Car Act, which required separate accommodation for black and white people on railroads, including separate railway cars. A group of 18 prominent black, creole of color, and white creole New Orleans residents formed the Comité des Citoyens (Committee of Citizens) to challenge the law. Many staff members of The New Orleans Crusader, a black Republican newspaper, were among the group's members, including publisher Louis A. Martinet, writer Rodolphe Desdunes, and managing editor L. J. Joubert, who served as president of the Justice, Protective, Educational, and Social Club at the same time Plessy was vice president.

The group contacted attorney and civil rights advocate Albion W. Tourgée, who agreed to help them bring a test case to court in order to force the judiciary to determine the constitutionality of Jim Crow laws. In his correspondence with Martinet, Tourgée suggested finding a plaintiff who had "not more than one-eight colored blood" and could pass as white.”

Rosa Parks

The Wikipedia articles on Rosa Parks and on The Montgomery bus boycott seem very clear that Rosa Parks decided on her own to break the law. Nowhere do they suggest that Parks was specifically selected to engage in her act of civil disobedience:

“In 1955, Parks completed a course in "Race Relations" at the Highlander Folk School in Tennessee, where nonviolent civil disobedience had been discussed as a tactic. On December 1, 1955, Parks was sitting in the foremost row in which black people could sit (in the middle section). When a white man boarded the bus, the bus driver told everyone in her row to move back. At that moment, Parks realized that she was again on a bus driven by Blake. While all of the other black people in her row complied, Parks refused, and she was arrested for failing to obey the driver's seat assignments, as city ordinances did not explicitly mandate segregation but did give the bus driver authority to assign seats.”

“During a 1956 radio interview with Sydney Rogers in West Oakland several months after her arrest, Parks said she had decided, "I would have to know for once and for all what rights I had as a human being and a citizen."

In her autobiography, My Story, she said:

People always say that I didn't give up my seat because I was tired, but that isn't true. I was not tired physically, or no more tired than I usually was at the end of a working day. I was not old, although some people have an image of me as being old then. I was forty-two. No, the only tired I was, was tired of giving in.
When Parks refused to give up her seat, a police officer arrested her. As the officer took her away, she recalled that she asked, "Why do you push us around?" She remembered him saying, "I don't know, but the law's the law, and you're under arrest." She later said, "I only knew that, as I was being arrested, that it was the very last time that I would ever ride in humiliation of this kind. ... "”

Although I haven’t spent much time looking, a few minutes on Google didn’t turn up any evidence confirming either of your claims.

Why do you think they are true?