The office of President wasn't created until 1990, and there was only one person to hold that office.
Are you thinking of the Premier (formal head of government before 1990), or the chairman of the Presidium of the Supreme Soviet (head of state)?
> Presidents where forced to resign, some where placed under house arrest.
The dictators (such as they were) were usually the General Secretaries of the Communist Party (including the three that were Premiers). Like most real dictatorships, rather than theoretical ones, there was a lot of power within the upper bureaucracy and shifting practical, if not consistent legal, checks on the power of the leader (particularly, the Politburo was powerful), and like most 20th century and later dictatorshios there was also a lot of political theater behind which the actions of the dictatorship were cloaked to give a veneer of legitimacy.
> Communism is Marxism. You will never be able to separate them.
Communism is broader than Marxism; as well as things like anarchocommunism, it also includes Leninism, which while it claims to be Marxism rejects several key components of Marxism.