I think the duckweed will survive fine, I've had some 'appear' in mine a couple of months ago, not suer if it came in with new plants or where it came from, but it's surviving fine and to be honest it's thriving in there! It is quite a pain to completely remove though, but will give some cover for for your fry.
I'd be more tempted to go for something like Amazon Frogbite as a floating plant. It's leaves are a lot bigger and the roots a lot longer giving much more cover. I've got some in the same tank my duckweed is in and not only does it look a lot better imo, but also provides much more cover.
I've not had the duck weed in my tank 'go wild' at all, it's been a lot slower growth than I'd expected it to be.