First off... in 600 AD, Guardia is ruled by King Guardia XXI and Queen Leene. In 1000 AD, it is ruled by King Guardia XXXIII and his wife was the late Queen Aliza.
Is "King Guardia" a title, or a name? As in, are all the Guardia kings from the same bloodline? And going by those numbers behind them, there have been 12 different kings in 400 years... isn't that straight up impossible? 400 divided by 12 is a little over 33... so in order for that to work every single king after Guardia XXI had to have passed on the throne around the age of 33. Seems extremely unlikely to say the very least.
Also... Lucca's little display to Crono shows that Nadia has descended from a long line of queens of Guardia. But... in both 600 and 1000 AD, it seems that the king was mainly in charge. And if Guardia is a name, then doesn't that mean King Guardia XXXIII is a descendant from a long line of male heirs all the way from 600 AD? Which makes Nadia a product of... incest?
Maybe I'm thinking about it in the wrong way. The fact that there's been 12 generations of kings in merely 400 years, when humans tend to live to 80-100 confuses me. The only way it works is if there have been some very unfortunate Guardia kings who died of extremely young age due to disease or being killed in action. But since it doesn't seem like there has been much conflict since the Mystic war, that latter one seems unlikely.