I'm writing up the combos section on floaties in the next few weeks, but there is a fairly comprehensive section on combo'ing fast fallers in my Lucas guide.
As for punishes, there is a variety of ways to punish. The move you use to counter your opponent should be done so that they receive the most damage you can possibly output, put them in a terrible position / ledge guard setup, or to take the stock. Punish game can include:
- MIs-spacing / bad approaches
- If the enemy is not properly spaced out when trying to attack you, simply being spaced back and punish whiffs or bad attacks on shield. Bad approaches include Fox trying to run -> SH Nair when they're not in range of hitting you. Punish by using anti-air Uptilt or angled Ftilt. Generally will be small hits that you get in neutral. Accumulated over the entire game, this could add up to a lot of damage and may be the difference between taking a stock or not.
- High startup / Long end lag
- This includes moves such as Falcon/Warlock punch, Forward smashes, Ganon Dair, Snake's reloading tranq, or an igglypuff after rest. These moves can be punished by fast attacks or grabs as well as simply shield or evade then run up and punish. Punishes on these moves are easier to perform and should be some heavy punish.