As long as you follow the flatworm exit directions, you should be fine. I have used it more than 10 times without a problem. The thing is, like andy mentioned, siphon out as many as possible before dosing. AND MAKE SURE TO HAVE WATER READY for a waterchange.
Mandarins do not really feast on flatworms. They MAY pick at them, but they can not survive on flatworms alone. So unless you have a very very high population of copepods and isopods, the mandarin will starve. I know there are some exceptions to this, but this is the general rule.
Even the wrasses are a hit or miss, I have a coris wrasse who could care less if there are flat worms or not. He prefers mysis over anything. lol