I've used King British Original Formula WS3 at half dose in a tank with dwarf chain loaches and amano shrimp. The shrimp hid for the duration but came out again after the med was removed. I discovered this one was safe with amanos as I couldn't catch them so they had to take their chance.
However, I don't know about the assassins. I removed my nerites before using the med. They are much easier to catch.