Sure, you can use Dominos to join the ends to the top, as long as you leave room for expansion/contraction with the outboard ones, and only glue the center one, just like is done on true breadboard construction. The downside of using only Dominos is you will have no stub tenon running the full width of the top, so it could cup between the tenons. Not likely, but that's why the stub tenon is used with true BB's.

I don't know of any rule for how wide the BB piece should be, but my guess is the 1-1/2 - 2" you are thinking about would be adequate. Aesthetics often is a deciding factor. And thickness plays a major role in preventing cupping so, at some point, the width of the BB is irrelevant if it's too thin.

John