On the issue of kyuubi, there is a lot of redundancy between it and a fox_tail multiple_tails search, but I don't think that is grounds for either nuking the tag or forcing the tag to only be applicable to images where exactly 9 tails are depicted. It may be worthwhile to require that the images have to depict a minimum number of tails to use the kyuubi tag though. Once you start getting a lot of tails depicted in the images, they become more or less similar and the general point of drawing many (even if not exactly 9) is to refer explicitly to the nine-tailed fox concept. As a suggestion, perhaps the lower threshold for what should be tagged as kyuubi as opposed to simply fox_tail multiple_tail be around 5 tails minimum to use the tag. Once you start hitting 4 tails or lower, the kyuubi reference feels a lot weaker and there is generally space enough to depict more than 4 if you're aiming to specifically reference the kyuubi imagery.
As for the bikini argument, I'd think even if the top or the bottom is missing, it should be able to classified as a bikini swimsuit as long as the rest of the image or outfit would imply the character is/was wearing the full swimsuit. Given as such an image like post #2879289, despite not showing the complete set, imo is properly tagged bikini because the rest of the image implies she's wearing a bikini swimsuit. Alternatively something like post #845054, despite only depicting a bikini top, shouldn't be tagged bikini because we know that with her complete outfit she's not wearing the complete swimsuit and that it's part of a non-swimsuit outfit. Otherwise I feel this will cause a big problem for tag searches like topless bikini and bottomless bikini, which imo are correct and appropriate combinations for searches.