No they can't. They're in different Egg Groups.
There is no cheat to get an Ekans in Pokémon LeafGreen version. You could try using an Action Replay though.
you have to go to route 11 i could be wrong but try
you went to a Nintendo event to get it on your FireRed/ LeafGreen or Emerald version OR you could get it from colosseum.
Trade one from Firered or Leafgreen Version. You could also use cheats and hacks,gameshark, or action replay.
Yes, many other Pokemon can breed with Vaporeon. So many that I cannot type them all here. (Well I could but I just don't feel like it). But you can find all the Pokemon breedable with Vaporeon on www.serebii.net. Just look up Vaporeon and scroll down to the very bottom and look at Egg Groups. It shows all Pokemon Breedable with Vaporeon.
Kadabra evolves to alakazam by trading it though it won't evolve if you migrate it to diamond from firered or leafgreen so what do you do? Get a kadabra in diamond trade it to a pearl version take it back and it will be a alakazam.
Unfortunately, you cannot catch Ditto anywhere in Pokemon Sapphire. To obtain Ditto, you must trade from Emerald. You could also trade from FireRed/LeafGreen
Shiny Pokémon are Pokémon that are a different color to normal. It would be possible to use an Action Replay to grant yourself a shiny Pokémon in LeafGreen version. You could get the code online.
No only firered can.
I can't find any listing for such a thing, are you sure you didn't spell it incorrectly? The closest thing I could find was a Pokemon, "Wobbuffet".
Yes
You could train in Victory Road. It has the highest leveled pokemon. Thanks very much.