I wouldn't try to hardwire the original fifteen Godzilla films (excluding GODZILLA'S REVENGE), there are too many conundrums and they were never meant to be scrutinized as such, much like the Bond films. The first string of Godzilla films operate on a very, very loose continuity, and I always have looked at Minilla (or Minya) in SON OF GODZILLA as an orphan adopted by Godzilla. If there was the first Godzilla, then a second (in GODZILLA RAIDS AGAIN), then there could be more. Also, if you see the uncut version of SON OF GODZILLA (released on DVD in the US by Sony), Godzilla is being led to Solgell Island by Minilla sending out "radio waves" -- if the creature was Godzilla's offspring, the King of Monsters would not have to be led to the island, wouldn't Godzilla instincts take him back to the spawning grounds?

Okay, that's way too much thinking about this...