I bought a ruby head fairy wrasse the friday after Thanksgiving of this year. It buried itself in the sand right away.....never saw it again. I actually considered it to have died, then a few days ago.....3 wks later.....it is swimming around in the tank as if it always has been there.
I am by no means saying that your fish are doing the same, don't know the tank, but, I will tell you that I consider myself experienced and have never had a fish just dissapear, and then out of the blue appear. Wrasses will bury themselves in the sand to feel secure, then slowly acclimate themselves to the tank, and tankmates. This was a first for me, but shows me that just about anything can and will happen in the saltwater hobby!!!
Good luck finding your two, as far as the other wrasses they should be fine with fairy wrasses as long as the tank is large enough to handle all of them. A 75g tank is very marginal for that many wrasses, considering the number one reason is that they all eat pods, and worms....eventually you will run out of the food they seek pretty quickly, and fights will break out for competition for live food.
You can feed them well, but their nature is to hunt for live food and this is why all four are not suitable for that size tank. If I were you, I would decide which two I wanted and stick with those. Of course, if is ultimately your decision.
