Duckweed isn't necessarily bad. It grows really quickly and can, if allowed shade the plants in your tank. If you have it and decide you don't want it anymore it can be a hassle to get rid of because if you don't eradicate it completely it will be back. Some folks around here have made a dam to confine it to a certain area in their tank. You find those that love it an those that curse it's name. So if you don't want it pick it out before you put the plants in. It would be the easiest time to get rid of it before it goes in your tank.
I completely agree with daFrimpster. The snails aren't necessarily bad either. Some claim they eat plants, but I have HUNDREDS, all the common species, and I've never seen it. I think they add an interesting dynamic and are fun to watch cruising around. They also help clean up extra food and can keep your substrate aerated in the case of Malaysian Trumpet Snails. The only real reason to get rid of them would be if you don't like how they look. They are not harmful.
I have Tons of both. I actually just cleaned my 55G of the over productive duckweed. As I don';t have any plants that need high light I let myself slack on the weekly cutting down on duckweed and frogbit. Well I got a 1Gallon Icecream bucket FULL now XD. But I still Love the Duckweed
snails cause no harm and I have them in all 3 of my tanks. MTS, ramshorns and pond snails
I like keeping my tank free of hitchhikers including common duckweed.
I take a large container of tank water, and isolate any new plants I get in there for a day or two. this gives me the opportunity to remove and identify foreign and dead plant matter, algae, snails, and bits of gravel. I keep the container next to my tank so it benefits from some of the light coming through the glass. When I sell my plants, I also isolate and inspect them carefully so others won't inherit any possible hitchhikers from me hitchhikers can be great! I got some Bacopa monnieri mixed in with some Elatine triandra I traded for, and really love that unintended plant. however I still like to take the time to separate and identify it before I put any unexpected plant in my tank.