Don't swords grow pretty big? Should be ok in a 40 gal but would not put these in anything smaller tbh.
Duckweed CAN be rid of as Steven F pointed out, I did the exact same thing, every water change and maintenance I spent ages taking out every duckweed I could see, a shrimp net is great for this btw. Until eventually there was none left, I was so pleased
I have replaced the duckweed with water lettuce, much easier to remove excess numbers, fish and shrimps do like this, provides nice shade and something for shrimps to nibble on
Personally I would go for java ferns, anubias and low crypts, LOTS of each can make a beautiful tank in a low tech set up