Where they a separate race as they claimed?
No. They just romanticide their past and called themselfs Sarmatians, anyway it was more of a lifestyle.
The Sweds romanticide that they were the decendants of the Goths since their name sound similar to their ancestors the "Geats"
Where they constituting a body of citizens, not unlike those of the Romans of old?
Yes they were the backbone of the P-L Commonwealth.
Maybe only nobles were Poles and the others were only Mazurian, Ruthenian,Zmudzian and the like?
The nobles in P-L were all equal... Atleast in paper.
I'm sure you prefer the national ukrainian dress of the time:
Made my night thanks!
In case of Commonwealth it was their rights, which made them standing out not only from other social group but from other nobles in Europe and the world. They had a rights not unlike Roman citizens of old, no matter poor or rich, every noble enjoyed the same rights. Freedom breeds a different kind of men.
On paper yes. But if a lets say Mr Potocki wanted the tiny little village of the poor Mr Piotrowski he took it by sword, and then bribed his way out of the court.
I would put the blame on the foreign origin of elected monarchs. They had no understanding of Polish constitution nor the will to work according to the guidelines of the law in Poland,
That's the reason why the Szlachta wanted foreign monarchs. Why have strong king with power that will make you his puppet while you can have a weak king that will be your puppet?