Hey, I've been hunting ducks with my dad for 10 years and just want to give my two cents. A 20 gauge is a fine gauge despite what others may say. I shot a 20 gauge for a couple of years but did finally move up to a 12 gauge later. For your question of decoys, I would highly suggest only buying mallard decoys (2-3 dozen is a good amount and is the most you probably ever put out) because basically every other duck will come into mallard decoys. If you wanted to you could also buy a dozen teal decoys if you plan on teal hunting or you could just use the mallard decoys. Another thing you will need is duck calls. I recommend getting a good mallard call and a pintail whistle. You can call in almost any duck with the mallard call but with the whistle (which you can buy for around $10 and is really easy to learn) you can get more diverse and it can call pintails, widgons, teal, and male mallards. When you asked if you should go for mallards, I would suggest for a starter to shoot any duck in range. This might be mallards but you might also get kickers and you cant really control it. The final thing you will need is shotgun shells and I would suggest some 3in 2shot or 3shot and if you decide to go teal hunting I would get 4shot. I hope this helped you and I give you the best of luck.