I've been thinking about this as well. I think that there is also something to consider: where does Natsu, Wendy and Gajeel come from? Even as dragon slayers, they all are perfectly fitted to each of their dragons. I don't think thats a coincidence. They are special in their own way and they seem to be the only ones to be pure dragon slayers, with all others having been tainted by lacrima or created by it. They seem to be the only ones who treat their dragons as parents, something that Rogue and Sting don't. Rogue and sting also seem weird when compared with their dragons. They aren't...exactly like them for the little we've seen and know of them.
I think Natsu, Gajeel and Wendy are different from Dragon Slayers in general and mainly because their dragons weren't, for me, your normal dragons. With this invasion we saw that Mothergea and the Jade Dragon....and the others... They weren't as amazingly powerful in terms of their abilities as Metalicana, Igneel and Grandine. We now know that Igneel is special amongst all Dragons. He was the King of Dragons. The king that was ruling the dragons until they disappeared and that was unable to defeat END. Igneel is special, thats a given. Now, Grandine and Igneel spoke as if they were "close" or something so we can assume they have some special relation. Its not a stretch to think that perhaps the 3 of them were the ultimate dragon leaders of their race, Igneel, Metalicana and Grandine.
This said, I don't think its a case of several individual dragon kings per type but rather that Igneel was called Flame Dragon King as Acnologia was called Dark Dragon King. A...epitome? Dunno the right word.
I'm still curious to know what happened to make Acnologia roam the world not doing anything. I mean, Igneel was at some point the Dragon King and so was Acnologia. Future Rogue said that Acnologia rose, sometime in the future, to rule the world again. Something ended its role in the past and somehow it stayed idle all these years. Then something happened and all Dragons disappear. Then Acnologia appears once again. And we now know that in the future, it tries to rule the world. And suceeds. That whole part of the story interests me plenty still. And I can't wait for this arc of random fighting to end and some more clues to be given to us. I like the fighting but I'm enjoying more the plot.