Playing floaties isn't about stringing long combos together, so you'll have to focus on baiting, punishing, and spacing. In the neutral game, throwing out an aerial is usually safe. Down throw, utilt, uair is your best option out of grab for stringing damage. Also, fireballs are really good. If you set up the wall, they usually have a hard time approaching.
For killing floaties, Mario has good options. F smash, nair, and fair (especially down throw fair) are very useful. Part of what may make down throw fair hard for you is that it kills much earlier (in the 70-80s on puff), and do it a lot. Probably your best option .
Edge guarding them is probably the most difficult part of fighting them. I wouldn't really challenge them off the ledge. Spam fireballs, and bait and punish.