1) not exactly true, useage of fingers has the advantage they need less area and the parasitics are smaller. disadvantage can occour when matching is the main viewpoint, with odd and small number transistors the mismatch is worse as I know if they are fingered.
2) I only agree with the short. It is not preferred to let any gates floating, if you short all terminals together to VDD(PMOS)/VSS(NMOS) it is guaranteed the device won't be open and won't consume current, very simple way to prevent it. With the source/drain sharing of dummies I don't have experience, I have never heard about it. I guess if you have to use not fingered devices for example for better matching, the dummies should be similar, that is why he said.