you could try a six-line. should work; i've heard reports that it doesn't work, too. same goes for a spotted mandarin. However, you'll need a tank that can keep it long term, unless you want to be able give it up after it clears your flatworms.
I've used the flatworm exit with success. However, as stated earlier, you probably dont' want to use it if you have plague-like proportions in your tank as the toxins from the dead flatworms could potentially kill any shrimps and potentially other livestock in your tank. If using the flatworm exit, give at least 3-4 days in a row, and then once weekly after for 1 month. blow your rocks with a turkey baster or powerhead to get the ones that are hiding. in addition, check for resistance. some may not die at the recc doses.
good luck!!