The main ingredients in "formagreen" mixtures are formalin (thats a nice trade name for formaldehyde, the stuff they use to embalm and preserve dead people) and malachite green (a dye that's been shown to cause cancer in some laboratory cases). It's a harsh med and the theory behind it is you kill the parasite before you damage the fish. I've used it and seen it used effectively in many cases, and I have personally used it with clown loaches and other scaleless fish at half dose. However, I would not recommend it especially for scaleless fish. Also, never combine any formalin med with salt as the combination is very toxic.
My real recommendation for scaleless fish with ick would be quinine. It is safe (ime) for scaleless fish and is an excellent ick med. I have treated tanks with clown loaches with it as well quite successfully, and it is also great for killing salt/heat resistant ick. Ick is becoming heat/salt resistant because it is such a widespread treatment (look how many people recommended it before me) and is becoming less and less effective in the same sense penicillin is now much less effective as an antibiotic. It can still work in a lot of cases, but it doesn't always work, and I prefer to kill external parasites ASAP before they get out of control.
Also remember to keep your water quality high, it's invaluable when having health problems in your tank.
