6-line wrasses are hit and miss with aggressive behavior. I have avoided them for that reason. they tend to get super territorial once established. they play nice with fish much larger than they are, but Mandarins are not larger and are really passive eaters. Chances are it will not get enough to eat, may hide often and eventually starve to death, unless your system is large enough for the wrasse to establish his territory in one area and the goby in the opposite side of the tank, say, 55 or larger.
Also, Mandarins are pod eaters, and need a large supply of food to graze on. Many do not switch easily to frozen fare. They are a tough fish to keep and need a well established tank to have a chance.
-Doc