Politely inform the Fish Store that they are perfectly, completely DEAD WRONG!
Just kidding.
FW Sole/Flounder are piscivorous. (fish eating) They will take to frozen blood worms, live black worms, plankton cubes and for fun use a small dip net in a local creek, stir up the substrate and net whatever floats just down stream. Dragonfly larvae, damselfly, stonefly, midges etc.
I keep three flounder (all within 3.5 inches) together in a 33gallon take. Their tankmates are oodles of gambusia, mollies, swordtails and platies. They have an almost constant source of fresh meat (fry)
Enjoy.
Respectfully,
FireMedic.