Call him Pac, it'll save your brain from a bit of trouble. That's what I do, anyway.
There are still the normal Power Berries - as shown in Episode 1 - The Adventure Begins (Sonic 4, anyone?), and Pac can still eat the ghosts without any additional powers, if the pilot shown years ago still holds as canon - it's just that it's so horribly ineffective, and that he can't enter the Nether-world without any berry powers. However, as the other berries come with added benefits, ones that the non-Yellows can use as well, it's more effective to use them.
As for the ghost gang, they're actually rather benevolent and not very evil at all (apparently - might back-stab depending on circumstances, but doesn't like Betrayus anyway), and they're basically helping Pac-Man out to get a chance to become alive and live in Pac-World again. I'm not even sure if it's possible in-universe. Anyway, Betrayus, on the other hand, is President Spheros' brother, who rebelled against him, lost, was killed and now is ruling the Nether-world.
...I know too much about this show than is healthy for me.