Hairgrass and swords should work fine, they are normally emergent or riparian species in the wild. Bacopa is a good choice; creeping jenny is another option for something spreading.
You should be able to find duckweed in local ponds or swamps, or if you don't like collecting from the wild, from pond plant supply places. They'll usually give you a bunch for the asking; it's a nuisance for them. Be careful with duckweed, though, as it can be hard to get rid of once established. A slightly larger floater such as salvinia, red root floater, or frogbit would be more manageable.