Changing people's religion by decree happened in many European countries. That was largely how the Reformation became established (especially in England and Scandinavia), and also how the Counter-Reformation re-established Catholicism in certain places such as Poland and the Austrian Estates. Of course in all these places there were some who privately adhered to their previous faith, but they usually didn't upset the public order. I don't think that would be the major impediment. The big concern would be whether the Huguenots would have the military strength to establish and defend such a kingdom, and it seems that they did, given that they held on to territory in southern France for a long time.