In The Republic there is a double layer of representation. At the top there is the Divine Idea, the Ideal, of "righteous person", and you might call this God's idea. An individual human being may attempt to replicate this Ideal of "righteous person" with one's own idea of "righteous person". This is the first layer of representation. Then the individual will act according to one's own idea, in an attempt to bring about the reality of actually being a righteous person. That is the second layer of representation.

For Plato all perfection comes from the Agathon (which is the Sun in the Parable of the Cave). The Agathon is therefore the analogous of "God" in a Christian framework (which I presume you are using when referring to God), and the idea of the just man (present in the discourse, most of all, in the example of Socrates, rather than discursively) is derivative in the sense that his justice comes from his yearning towards the Agathon. Remember that for Plato all persons live "in the middle", in an intermediate field between ideas and corruptible matter; and our knowledge of ideas comes from having beheld them in a prior existence (i.e., not in an existence comparable to our current, material one).

So, no, the just man (and any other idea) is not superior to God. I think the best that can be said is that ideas are aspects of God, but that formulation would never result in them being superior, or not even equal, to God. They are, after all, limited, while the Agathon is not.

Presumably, Plato would say that God isn't a person, per se. So comments about whether there can be an "ideal righteous person" don't apply to him. God would rather be, or at least exemplify, the ideal.