Re: So, only the constitution is valid?
Many state constitutions are rubbish. This is especially true, I suspect, in states like California where they can be amended with relative ease by ballot initiative. Compared with these, the US Constitution, with fewer than 4,400 words to describe the basic structure and functions of the government and the authority of the main parts, is an elegantly concise work of art.
As an aside, the original US Constitution made no explicit mention of slavery. The closest things to that were Article I, Section II, paragraph 3, the three fifths rule that allocated representatives based partly on "all other Persons," a category comprised of slaves; Article I, Section IX, paragraph 1, prohibiting Congressional action to end the slave trade before 1808, and Article IV, Section II, paragraph 3, requiring a "Person held to Service or Labour in one State, under the Laws thereof, escaping into another" to "be delivered up on Claim of the Party to whom such Service or Labour may be due," a clear, although implicit, reference to slaves. Looking back, one may consider these compromises contemptible, but the existence of the Union arguably depended on them.
On the other hand, the original Constitution said exactly nothing about the qualifications of electors, that being left to the states, and nothing about the qualifications for federal elective office beyond age, citizenship, time of residence in the US (president and vice-president) or state from which elected (senators and representatives). Women and black persons (who were property owners) were electors in the state of New Jersey from 1776 until 1807 and therefore eligible for election to the US House of Representatives. It would have been unconventional to elect a woman (or black) to office, perhaps to the point of being unthinkable, but the US Constitution did not prohibit it.