I tried the velvet nudis for flatworm control once, and my general opinion on something like that is that unless you can get the predator to breed in your tank (which I don't think velvet nudis will), they offer little hope of putting much of a dent in a pest population. If one velvet nudi eats even 200 flatworms a day, it's still not going to be enough to do much to an out of control population.
the same thing is true for berghia's as an aiptasia control method. Even if you buy 20, they only really start to work when the second and third generations mature and the population reaches critical mass, as you need hundreds of them working together to outpace the reproductive rate of their target pest.
I think you made the right choice with flatworm exit. It's extremely effective at doing what it does, and there's ways to mitigate the risk. If I were you, I'd do a second treatment a couple of weeks later as it's almost inevitable that you'll miss a couple if the starting population is really huge. A second dose soon after poses much less risk of flatworm poisoning and will mop up any surviving worms.
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