I would encourage you to do this- it's wonderful to have a bunch of little bitties.
The ones with feathered legs will need ramps to get to low roosts. The ones without will want to fly to very high spots to roost, like rafters. Those without feathered legs have the ability to fly like birds, for 50 feet or more, and up high. It's important to consider this if you're in-town. They will want to fly into your neighbor's yard. Guaranteed.
The eggs vary from malted milk balls to very small eggs. You will want to get 3 birds for each egg you would buy at the store: If you eat a dozen eggs a week, you'll want to have 36 eggs laid per week from banties. Many of the banty breeds are still prone to broodiness, so if you get Cochins or Silkies, even Seramas, you can expect them to brood 2 or 3 times per year, so they'll be out of laying for 8 weeks or more.
All this being said, most stand their own with the big girls, are wonderful personalities, and I love all of mine dearly. I have Bantam Salmon Faverolles, d'Uccles, Banty Cochins, and Silkies. I adore all of mine and think they're just as wonderful as my giant breeds...and I have a dozen of breeds of those...