Gypsy is Slinky, Bunny (she's super soft, like a rabbit), BunBun, Slinky Buns, and SpazMo G (her rapper name).
Gracie is Baby Grace, Fatty, Chunkers, Fat White Lump, Poohead, DumDum, MooMoo, DisGrace, and her rapper name is Heavy G. Really, we love her, but she's so chubby and kind of dim.
The kittens haven't been around long enough to get a lot of nicknames, but Pippin is Pipsqueak (I actually started calling him that when I first found him), Pippers, and Pippy (Pippy Short Stocklings, since he has white toes).
Fergus is Fergie and Ferger-burger. He's a little cross-eyed and a little slow, so he's sometimes Dunderhead.
I'm sure as both kittens' personalities emerge, they'll get more names. As a pair, they're Kitten Littles, as a play on Chicken Little.
Beavis is Beefy, Beefcake, Grandmaster B (they all end up with rapper names!), Dookie, and a variety of names I can't repeat here when he misbehaves.