I'm not so deluded that i think people are good and nothing but good and only capable of good. People are tribal. They are covetous. They are jealous. They are aggressive. They are violent. They get sick. They get stressed. They get cancer. They start fucking the others girlfriend. Mental disorders arise. Psychopaths routinely and successfully fake personality tests. Stress changes every one. Millions of things happen to change personalities, outlooks, mental states and trigger discontent and ultimately violence. Some people turn out to be perpetually lazy, some very hard workers. Everyone has a subjective idea of "fairness" that differs greatly from that of others.. Confinement ultimately brings out the very worst in people... just as every prison, everywhere ever fully demonstrates. You cannot predict who will lead and who will follow. You cannot predict uniform agreement and consensus on life altering decisions. You can easily predict discontent, disagreement, resentment, anger, jealousy, etc etc etc.
All that we are as people in terms of our behaviors, are constantly evolving and taking twists and turns which cannot be predicted. People can't just be voted "off the island" in this case, for developing problems with others.
You cannot predict how a person in today's state of mind will be behave in a future state on another planet or predict it at any given point of time. You just can't. It is not possible.
It's not even reasonable to predict positive future mental states under stressful and completely new conditions any more than you can predict the weather 1 month out.
You cannot predict how they are going to respond and react to leadership and each other when they are faced with countless life threatening events and life altering decisions that have to be made as a consequence. It only has to go wrong once, for everything to devolve into chaos.
There is no difference between 20 people and 100 people or 200 or 2000. These people, in whatever limited numbers are going to be confined, restricted in movement and locked up, unable to escape the company of those around them for the rest of their lives.
Furthermore, only people that would choose to die on Mars aren't exactly going to be the most stable personalities to begin with. A stable, normal, well adjusted person is one with a great career, a great marriage and a great family and friends. It goes against our biology to walk away from that as an emotionally healthy, well balanced and centered human being (no offense to your nomadic lifestyle and living in a vehicle). Someone lacking all that is undoubtedly dealing with personality problems that will not serve everyone well.
Eventually, primitive behaviors take over and people act like the evolved chimps that they are. This is always the case, everywhere... on the planet. It makes no sense that people think that in a world full of people who happily stand by and watch genocides unfold, watch children get raped, crucified, beheaded or sold as sex slaves are going to somehow be model citizens under the most extreme circumstances, every experienced by mankind.
Just having a "positive attitude" and "believing in the dream" isn't going to overcome the practical issues of dealing with the dark side of human nature.