You can be a Christian capitalist, or not.
You could argue that voluntary socialism is in line with Christianity, even called for...it is voluntary charity taken to the maximum extent. Many early Christian communities were this way, as were many religious orders.
I would, however, argue that capitalism is more allowable by Christian doctrine than enforced socialism, though. Capitalism allows for free conversion to charitable principles (and therefore conversion of the soul...of both the giver and the receiver). Enforced socialism/communism does not allow that, and indeed it seeks to do away with charity altogether (allowing for no conversion of the soul).
Catholicism has rules, but people are free to disobey them. The Church should only use persuasion to abide by them, not the force of secular authority. Whenever the wall of separation of Church and State is breached, and the Church gains secular power, the Church loses the moral authority of persuasion. Governments use force to coerce behavior of their citizens. There should be limits on the topics governments address through force; there is no reason to limit the persuasiv power of the Church...it is just free speech after all.