I assume they would go for HQ-16 on the 052B because it has the Buk which is the HQ-16's ancestor. And HQ-9 for the 051C in line with that ship's original role as an area air defence destroyer.
Yes, they would match the missiles to the closest of what's already in there. The 051C presents a problem however, because the Type 346 Aegis like radars are used for guidance of HQ-9 missiles, and the 051C cannot fit those. Somehow you have to make the HQ-9 work with the Russian Flaplid radar that's already on the ship, which means retaining that radar or navalize the land based HT-233 radar just for the use of these two ships, or you have to create a special version of the Type 346 radar just for this ship. The easiest way is to bite the bullet and procure new S-300 missiles from the Russians and have the Flaplid radar updated, while the other Russian radars on the ship, namely the Top Plate and the Bandstand radars are replaced with their Chinese equivalents. Given that China recently bought S-400 missiles, that means they still have a current working relationship with the manufacturers of the S-400 missile systems who also did the naval RIF-M system on the 051C.