I don't think I have ever seen bluebills and mallards sitting together in real life.

Mallard decoys will work for just about anything. If you want some color in your spread, get you a seagull. There isn't a duck the Lord has made that will not sit down next to a gull.

Floating Mojos are great and all... but you'd best be prepared to mess with it a bit.. they take a heck of a lot of adjusting and in my honest opinion, after 6 years of using one and having gone through 3 of them now... they are almost more trouble than they are worth.