That's consistent, since a modern PLC will have no monarchs and crowns.
Hmm... maybe joining in the flags of Ukraine and Belarus would make the flag a bit more colourful. I mean, Minsk and Kiev were also major cities within the original pre-partitioned PLC territory.
Well it does not matter - the crown means soveregnity, and it does not matter if it is Republic or Kingdom...
Also, the PLC was republic btw... Respublicae duo Nationes. The Republic of both Nations. Which mean the sovergein nations were Poland and Lithuania, joined by the union.