For a very small population, I wouldn't worry at all. The chemical is harmless. The risk with it is if there is a large number of flatworms dying, they release toxing that have wide ranging implications. By large populations, I'm thinking numbers like 10-20 per gallon - perhaps much higher, but I've always just eyeballed the tank and made a judgement call. My first time using it was in a tank that was blanketed with fw. A 12" wide toadstool leather had a maroon jacket of fw on the whole trunk. I removed the fish and the corals were fine - but I did a 50% wc after an hour or so then ran fresh carbon.
Any predator solution can be hit or miss. Personaly, I'd dose you tank and be done with them. Repeat in 10 days or so if you want to make sure there are no eggs that hatched. I've got one tank in my care that has become a real conundrum. The FW are drug resistant after dosing with Salifert and Flatworm Rx. The tank also has a pair of flame hawk assassins. They don't like newcomers and have killed spotted mandarins, scooter blennies and a 6 line wrasse. YT damsels survived but they only occasionally eat fw. One note for anyone using Flatworm Rx by Blue Life is there a misprint in the instructions. They give 2 dosages that are off by 10 fold. Consult the company for proper dosing - they are aware of the misprint but haven't done anything about it. That is likely where my drug resistance started. I thought I was approaching 10x dose and found later that I was only approaching the recommended dose.