yes both are aggressive, Cape Barren will pretty much kill anything in their pens, and they especially hate shelducks.
First you have to find Cape Barrens though before you even need to worry about it. Then you need to be able to afford them when you do. Only a small handful keep them in North America.
Being that you are just starting out with these type of waterfowl, I'd highly advise against even worrying about all these. Stick with the common, cheap, and easy to find and keep species first. Then if you decide youre good at it and enjoy it, move on up.
any of the common teal, wood ducks, mandarins, pintails, red heads, widgeons, etc. They all make good start up birds.
if you like the shelduck, go with common/ European, or radjah. They are both way more compatable with a community group than the others