R2 is the feisty one, should be RD.  
Twilight and Luke have the most character development and the most power in-universe.  
Rarity is C-3PO, mostly because of their prissiness; Leia is more bossy than Rarity, not nearly generous enough for a match…then again Rarity and Leia are never damsels in distress. I guess Rarity is the closest to Leia.  
Not sure if Spike or AJ would make a better Han. Han is supposed to be modeled after cowboys in old film serials, and he’s never as well mannered as Spike is. Han’s too much of a prima donna sometimes. They both have strong enough egos and tend to get greedy, but put their friends first when it counts. Yeah, Spike is Han.  
Fluttershy is C-3PO, because he’s the most kind, even when it’s bad guys, but can get frustrated more at other ponies than at things that aren’t her own species–C-3PO is that way with R2 and lower life forms.  
That leaves Pinkie as either Lando or Chewie. Hmm…Chewie Pie is almost too easy. Chewie can get whiny sometimes, so he’s not a good pick for AJ…he’s really loud, so maybe one of the CMC. Pinkie is probably best for Chewie since he’s more sensitive to mood swings.  
That leaves AJ as Lando–friendly, loyal, outgoing, plays her cards close, and patient. Sometimes might have ulterior motives like money.  
Ben and Darth Vader are Celestia and Luna, no question–Ben one in a big deciding fight; only difference is the Nightmares never one out for more than a short period in history. Probably be one of the big bads, like Sombra; Chrysalis is a manipulator and has long cons, so she’s a better Emperor.
 
Now someone do the prequels. :3

Spike should be Han Solo. Rainbow can be R2-D2. Fluttershy being C-3POc

@Trickquestion  
chewbaca XD

@Background Pony  
It’s not as obvious as AJ or Spike, but she’s in the same place in the line-up as 3PO, especially at 0:34. Besides, she’s the fastidious one with the elegant accent.

@PlotHole  
I don’t see where Rarity coincides with C-3P0.

The show itself went with:
 
AJ = Han  
Twi = Luke  
Celestia = Leia  
Spike = R2  
Rarity = C-3PO  
Pinkie = Chewie
 
They were trying to map 8 characters onto 6, so Dash gets a little Han, and Fluttershy gets a little Chewie (you can write your own joke on that :p).

@Background Pony  
Cultured accent.

@Trickquestion  
Why does everyone always associate C-3P0 with Rarity?

Twilight: Princess Leia  
Shining Armor: Luke Skywalker  
Spike: R2-D2  
Rarity:C-3PO  
Rainbow Dash: Han Solo  
Big Machintosh: Chewbaca

@Merovech  
Fluttershy would be C-3P0.  
Because of reasons.

Why the hell isn’t Pinkie Pie Chewbacca?! Also, who would C-3P0 be?

@Dumb Rock
 
Yknow its funny. I think Flufflepuff would make a great Chewbacca

y’know, with the phallic imagery that unicorn horns already have this is so more fitting for spaceballs than it is for star wars itself.

@Blackbewhite2k7
 
Chewie? The black thing on his shoulder may be his bandoleer.

oh, hi twi…  
and then, she died

I agree entirely with the casting here…but who is spike supposed to be?