How do we know the courier was skilled?
Exactly, The Courier doesn't have a back story at all. And in FO 1 & 2 your small guns skill would start out higher.
Fallout 1: 35 + Agility
Fallout 2: 5 + 4(Agility) (I typically start with around 45% without tagging or Gifted)
Fallout 3 and Fallout: New Vegas: 2 + 2(Agility) + Luck/2
Also too the Vault Dweller had a party of 4 (Vault Dweller, Dogmeat, Ian, Tycho) and the Chosen One had a party of 5 (6 with a bug on V 1.0) (Chosen One, Sulik, Vic, Cassidy, Marcus, and other [Goris, Dogmeat, Myron, and Mirion/Davian]) so I'm pretty sure they could gank the Lone Wanderer and the Courier.