Yes, he is the most sadistic class, but he is also the one who engages in actively damaging the enemy least often. There are really no characters that are grim sadists in the show. There are several that moderately fit every other class.
RD has the cocky, speedster attitude of the Scout. The Heavy appears to be a ditz, but has a deep mind, while harboring a protective sense for his allies, while being immensely strong, which fits work horses with a strong family bond like Big Mac and Applejack. Applebloom is a fairly clever pony, that has some talent in building things, which fits more or less with the Engineer, who’s a genius with the know-how to operate powerful buildings. The general thought that Twilight is a Sniper and Rarity is a Spy kind of irks me, considering the personalities of the characters. Twilight’s organization skills, wits, and knowledge strike me as decidedly useful for espionage, while Rarity seems to be the kind to avoid getting into the grime and mud, staying up above to aid her team with precision and finesse.
As for the Demoman, Pinkie Pie seems to fit him, what with the chaotic nature, rambunctious attitude, and tendency to cause mayhem and destruction. Zecora might fit, even though he’s Scottish, but I don’t see Berry Punch being an awesome fit (that’d be like using a sandwhich pony for heavy, based entirely upon the sandwhich). The Pyro doesn’t really have a set personality, too much, beyond liking and being immune to fire. Spike more or less fits that, in my opinion.
Finally, we have the Soldier, who I’m iffy on. I picked Shining Armor because they’re both strong protectors who have some level of patriotism, and a protective nature of what matters (MY intelligence!).
For the NPCs, I think there’s little debate, there. The Administrator pulls the strings from behind the scenes, something that fits Discord wonderfully. She’s also cold, cruel, and is not opposed to murder or inciting anger and rage among friends. Luna and Celestia are sisters who have their differences, something that fits for sparking a war between them, a la Redmond and Blutarch. Iron Will’s a bold, masculine salesman with a boisterous attitude, making him a perfect Saxton Hale.
Wow, look at all my opinions, they’re just falling from the sky. It is an opinion, really, so I can’t back it up to much, beyond pointing similarities out. Medic is mostly non-combat, assisting allies and being supportive, something that fits Flutters. He may be a sadistic son of a bitch, but there’s really no pony that could fit that (Pinkamena doesn’t count).