The Magikarpachu stage isn't that bad IF you know the move to make. Move the Karpachu from the first COLUMN to the middle of the first ROW, where the Pichu icon is. (A4-C1)
This triggers Karpachu's Swap++ and clears the stage in one move, giving you a solid 40% catch rate.
I would probably have beaten the stage easier the first time if I had known that Karpachu's ability was Swap++ (it's the same as Magikarp, I know, but do you think I knew Magikarp's default ability off-hand?)
I've encountered everything a couple of times. Only Shiny Magikarp got encountered once (and I failed the stage), but I've caught everything, so I'm only doing the Safari because Exp Boosters is mildly more rewarding than Meowth, since I'm already at 46k gold, and the Cloyster stage, if drawn, gives Ampharos 100-200 exp (everything else I'm using is maxed).
Wish I'd checked this to see what the move was. Was still able to get the catch with a single great ball and a lucky super catch, but better odds are very nice.
I honestly value the gold more than just about anything in game and won't stop grinding until I'm in the 80K range. As for the experience candies I'd rather just play the Ampharos stage. Or slowly grind a Pokemon on Mewoth.
The experience candies are at least decently common. I'm at 9 small, 2 medium, and 1 large.