Yes and no to all of your questions...

Brian Boru buttons tend to be round (with little harps on them), although I have seen them with square buttons.

Prince Charlie and Argyl jacket buttons tend to be square and come in 3 different sizes (small, med and large). Some jacket makers use both med and large buttons for different parts of the jacket.

Also, they can come in Chrome and Antique...

Hope the info helps...