Pleasure is the global term; tranquility and ataraxia are fully contained within the word pleasure, but "pleasure" is not fully contained within tranquility or ataraxia. There are pleasures which do not involve tranqulity or calmness or any other similar term. Are those other pleasures less "worthy" than calmness?
I wonder if the idea of "mixed" pleasure might need some further examination because it seems that there may be pleasures which are mixed with a tinge of mental uncertainty. There are times in life when you chose pleasures in which you are uncertain what the final result will be (mental pain or a minor problem may result but you are fairly confident that you won't end up physically wounded or dead).
And thought that came to me regarding "pleasure is the absence of pain" is that this is simply a guide or a tool to find moderation...rather than arbitrarily deciding "I will only eat one heaping full plate of spaghetti" then if you use the phrase "pleasure is the absence of pain" to decide to stop eating when you aren't feeling hungry anymore.
So it is moderation based on sensation and not on abstractions.