Are plie squats good for toning my inner thighs?

Not really. Of all of the muscles plie squats work, inner thighs is probably the one it works the least. By toning, people usually mean "lose the fat from a particular area." Now the question becomes "Are plie squats good for losing the fat from my inner thighs?" No, they are not an effective exercise for losing fat from the inner thighs. Resistance training works muscles. Muscles do not draw exclusively from the fat over them when they work. Working a muscle can strengthen it, increase its size, or improve its stability, endurance, or other performance parameters. The one thing that will not happen if you work a muscle is disappearance of the fat covering it. The only way the fat will go away is if the calories you eat are less than the calories you burn. Even then, you cannot choose when or where the fat will reduce. If we take this question a different way and look at the plie squat as an inner thigh exercise, it falls short again. The position one takes for this exercise puts the inner thigh, your adductors, in a lengthened, and therefore weakened, position. As you descend into the squat, you are stretching the adductors even more. The muscle starts lengthened and becomes even more so, never really shortening. One could say this exercise makes a better stretch than a muscle builder for the inner thigh.