On the subject of whether the others were friends before meeting Twilight, here's how I see it:
I can see Rainbow Dash and Applejack being
Vitriolic Best Buds, as they have a lot in common and just have a dynamic when they interact, and with the fact Applejack knew about Rainbow Dash's Sonic Rainboom when she did it the first time. I don't really see either of them as having been friends with Rarity or Pinkie Pie, as RD/AJ are basically polar opposites to Rarity and Pinkie Pie is just such a handful.
Fluttershy obviously would have trouble making friends, and I doubt whether she even knew Rarity all that well before as, if they were friends, it would seem strange for Rarity to be so surprised that Fluttershy had such knowledge of sewing in "Suited for Success". She may have known Rainbow Dash on some level, but I doubt they were friends at all; if anything, Rainbow Dash seems to rather dislike Fluttershy.
If we take Pinkie Pie on her word, than she may indeed know every pony in Ponyville. Knowing her, just meeting someone and throwing them a super-special-awesome party is enough to make them her friend. I can't really see her really sitting down and getting to know anypony though.
edited 28th Mar '11 3:55:24 PM by Dragon_Ranger