I think you may be overstocked but I don't calculate these things by the inch per gallon/ square foot rules.
Your plec needs to be rehomed or you need to get him a much larger tank, putting him in a 10gallon, as wrs has said, will stunt him but not only that the levels will be terrible as (if, more like) he grows because they create so much waste.
You may not be overstocked once the plec is removed, depending on other factors. Stocking shouldn't only go on size of fish but also how much was they create, what space they occupy (it's possible to overstocked a particular area of the tank), your filtration and what your levels are and how many water changes you do to keep them right.
I do not know about the golden shark, do you know a more helpful name for it? I am quite surprised that your ruby shark has not been going after it, these are know to go after other fish that are even shark shaped!
What are the parameters of your tank? How much maintenence do you have to do to achieve that? Does you nitrAte rise higher in a week to be more than what it was when last tested (hope, I made that clear enough). What is the nitrAte level of your tap water (otherwise knowing the level of your tank is useless). How much filtration do you have, as in how many litres per hour does it do?