You are correct. I have read stories about flatworms getting to plague proportions, but I have not experienced that. And I attribute that to my wrasse. I thought about running some flatworm exit, but I have about 350 gallons in my system, so I would have to dump in a LOT of that to make a difference. Maybe I’ll take my 40 fuge offline for an hour or and dose the flatworm exit. But that'll only get rid of them in that tank. I won't be able to guarantee they'll be gone everywhere unless I treat the whole system.If the 6 line eats them then that's a great free food source. Many years ago I had a dozen or so (clear ones) and seemly overnight they exploded into hundreds and they were everywhere. That was in a tank 9-10 years ago, but I seem to remember a mix of white and red flatworms... or maybe two different epic plagues.
So many that they were obstructing much of the glass to see into the tank and the water column pretty saturated as well.
I don't remember anything "bad" happening other than clean-up was a bear. At the time, I really only had softies, a nem, and a few hammer/frogspawn/etc in the tank.