On the issue of Ghost-types, perhaps they're the spirits of a now-extinct species of Pokemon (Later games mess up the "dead pokemon" perception as you can breed them, leading me to believe that they're simply a species of Pokemon that were initially mistaken as ghosts, and the name stuck). Remember that the Anime completely disregards many aspects of the game itself (such as in the first movie, where Ash's Pikachu takes out 4 Pokemon - including a rock/ground type - with a single thunderbolt.
For the Baby Kangaskhan, I can't really comment.
Finally, on the subject of Mew, there are only 4 ways to get it.
1. Nintendo Event - long since discontinued (Basically you'd plug your game into a unit which would add an Event Mew onto your cartridge) 2. Link cable Trade - The only way to obtain a Mew without the use of external game-altering devices or glitches. 3. Cheat Cartridge - Like the Nintendo Event machines, these simply let you manipulate your cartridge data, but allows more freedom to do what you want rather than adding a single "traded" pokemon. 4. Mew Glitch - The only way for most people to get a Mew nowadays.
Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming