Just for testing I bought some female swim thongs, two rather small ones and a larger model. The small ones did not fit at all, the balls were dangling outside the pouch. The one with the larger front fitted better. I had to pull the front down to have the wider part of the frontal triangle to cover my equipment. To wear this thong at the beach, I had to shorten the strap between the cheeks. With more than 150 male thongs and strings to choose from, it was too much effort.
For wearing a female thong at the beach you need something with a wider front and reposition it. It might work but I don't know if it is as comfortable as a male thong. Finding the right form will be trial and error.