There are two swimsuits from that moonfire fair, both have the appearance of a skirted one-piece swimsuit (not a bikini), although they come in two parts.
Despite coming in two parts, they are most assuredly not bikinis, although they *are* dyeable. They are nice, and I have them both. But I would like to see at least one plain bikini set that can be dyed, and that has both a top with straps and a strapless top.
That said, in many ways the Hempen set that the OP posted about does the same job - depending on the style - as a plain bikini would.
Allowing the different styles of Hempen top and bottoms to be used by everyone would fix things, and it should not be a major issue, given that most armor simply scales to the character anyway.
Actually, take a look at these images;
There are two designs for Hyur one of which is uber strappy, the Elezen one is incredibly elegant, I love the Roegadyn look as well, the Lalafell set would make great sleepwear and the Au Ra one quite frilly. Opening all of these up to all races would massively widen the wardrobe choice. I only included the female ones, since Kos is female, but I am certain that there would be a similar increase in the choice of wardrobe for male characters as well.
P.S. Please support the OP and like their post, if the developers see this topic/request and it's support, pehaps they can make it so.