Maybe it's just me, but I have trouble in large assaults with Skirmishers. Skirmishers are great at defensive warfare but I don't have the kind of success invading other civs that others seem to have. Part of the problem is that it's very hard for me to siege and take down cities, and enemy roads inland somewhat nullify my mobility advantage (I can't spend a lot of

pillaging their roads). I prefer old-fashioned Siege + Melee for early rushes.
Offensive warfare can start as defensive warfare though, which is a plus for Skirmishers. Let the AI advance, kill their units, then go in. I just can't rush that effectively with Skirmishers.
Frigates are really good for rushes, though.