If you want to get rid of the FW, not a bad idea to get a six-line. However, I'm not sure if you want to keep him there long-term. I've also heard that six-lines are hit or miss.
I haven't tried a mandarin or a six-line myself, only what I've researched when i was trying to rid my tank of FW.
From my experience, using the Flatworm Exit was very effective for me. I haven't had any FW in 6 months. However, just like any medication, if you're going to use it, you have to use it correctly. Otherwise, you can kill any shrimps or other sensitive creatures from the toxins released when the FW die. Key is water changes and physical removal. The other problem with the medication is if you don't dose correctly, they'll just come back. I was dosing once a month and they simply came back the next week.
Melev's web site is very good.
http://www.melevsreef.com/flatworms.html
If you're going to dose aggressively, try the following. This what worked for me.
What I did to rid it was dose for 3 days in a row with corresponding water changes (at minimum 25%) and running carbon. Then, dose once weekly for 1 month (with water changes and carbon). Take a powerhead or turkey baster and blow your rocks to hit low flow areas. In addition, take a brine shrimp net to get the ones floating in the water. It seems that the Flatworm Exit will kill some within the first 1 minute. Others, it will take a couple of minutes. The more resistent ones, it will simply stun and you will see them running to a low flow area. This is where you use your turkey baster and net to get them.
Good luck. I truly hate the FW. Because of them, I dip all rock and coral in saltwater that hs Flatworm Exit. I then do an diluted iodine dip per Tyree method.
