Are all humans in Heaven equal or is there also a kind of hierarchy? (Roman Catholic perspective) If so what determines what their position is? 

What is equal is that all human would see God from face to face. But Jesus made an implication that one could sit on either of his site. Are these place for special holy persons, like saints?
On the other hand some persons go straight to Heaven and others will first go to purify themselves in the purgatory. So from that site one perhaps can expect that after purification every one is equal.

So is there a kind of hierarchy in Heaven, if so what determines it?

This is according to RCC.
No, everyone in heaven is not equal because of the diversity of their merits.

The degree of closeness to God in heaven depends on one's merits. The Council of Florence, under the authority of Pope Eugenius IV (1431-1447), defined the dogma (DZ 693) that


  the souls of those, who … are immediately received into heaven, … see clearly the one and triune God Himself just as He is, yet according to the diversity of merits, one more perfectly than another.


Since the Blessed Virgin Mother has the greatest merits, she is the closest to God in heaven.

Also, one cannot merit in purgatory. Since people die with a diversity of merits, so is there a diversity of merit in heaven.