Thanks for the feedback, so far. I did add a sixline to my tank this week (no quaratine). I fed him some of the flatworms during acclimation just to see if he'd eat them- he did and is doing well in the tank. He does hang out where the infestation was at its worst (not bad really). I think there are fewer now, but my wife disagrees.
I didn't realize how much I'd like the sixline- beautiful colors and a great personality (I swear he's lookin at me). He doesn't hide too much like I'd expected.
The other thing I noticed is the cleaner shrimp picking over the rock with the flatworms. I can't tell if he's eating them (his legs move to quickly) but it seems as if he is.
Regarding the velvet nudibranch... I think I'd do the Flatworm Exit before I'd try the nudibranch, only because I suspect it would ultimately starve in my 40 gallon tank after the flatworms were nocked down or out. Same for a Mandarin, though they're about the coolest fish ever.