Are they less likely to be executed? And are smart routing algo's used by brokers "show" hidden orders to all exchanges (despite the fact that they are hidden)?
The other side can snoop out hidden orders with 1 share pings. Hidden orders are last in priority as well.
if it is hidden order, e.g., on the island exchange, 1 cent ahead of the nbbo, someone sends a limit order to nyse to lift all shown nbbo orders, how can nyse exchange know there is hidden order on island if island does not have nbbo orders?
I would say 'yes" because of the fragmentation in the market. Not only that, but you can have a trade through on a hidden order as it is not protected with Reg-NMS. Not to my knowledge.
Hidden orders gets executed 100% of times if some one takes/hits on the same ECN (of hidden order). However if some one send an order on other ECNs , hidden order may or may not get executed depending upon the routing mechanism. Open orders on NSDQ,ARCA EDGX and BATS will most probably execute hidden orders. Smart orders are mostly hidden orders and are not shown.