i dont see why not. only problem with duckweed is if your are trying to grow plants below it for lighting and nutrient purposes. otherwise, it will not hurt the fish.
if any of your lfs has is, just ass if you can have some, im sure they wont mind at all. but if you have a ittle room up top, id get something a little cooler that grows slowly like water lettuce or frogbit
Duckweed should be free at any pet store that has it, if you are talking about the tiny annoying stuff that sticks to your arm every time you put your hand in the tank.
OH and I did have the regular duckweed and larger duckweed in with juves at one point... The did well The filter ate them.. But until then they did well..