Miqo'te Tails don't stick straight out, they point downward. If a costume has even the tiniest bit of extra space in the butt area, you can maneuver the tail after a certain point down one leg of the costume.

If your argument against hiding tails with certain gear, such as the Chocobo Suit, is that Miqo'te tails are shaped incorrectly to be hidden that way, you're just wrong. If you were right, there would be zero equipment in the game that hides the tail, but there are items that do hide it. I know, for one, that the Wolfseye Robe hides your tail and only the fake wolf tail is shown. Also, the Evenstar Coat? That can't be my tail, it's far too thin and permanently erect. That has to be a part of the coat itself (and someone's idea of a speciesist joke since they only put it on coats they give to Miqo'tes), and my real tail has to be hidden somewhere.