Woah, woah, woah. Alright, let's all take a step back. As much as I totally agree with the rest of you, especially since I greatly dislike Ash, and as much as I did originally plan on agree with you all, it really is not at all that bad an idea. I don't know about giving him a pokedex, as I don't even see him to be worthy to be one of the chosen pokedex holders, but it would be logical for him to receive one as well.
We are all willing to argue against it, so I can pose a few things for it.
First off. If one were introduced to the other, I believe its Ash and Pikachu to the world off Red and Pika, it'd first have to be picked which trainer is older. Which of the two would have started their adventure first? If he was from Pallet, received a Pikachu from Oak, and began his adventure with/without a rival, the younger would/could/might be informed to the existence of the older without the older becoming aware. So. Since the reality of what is Ash and Pikachu collapse and conform to the world of Red and Pika, it is only left to assume that Red would have to older.
Why? I think, as I said I only think so others could argue differently, that since Ash/Pikachu has enter the world of Red/Pika, that Red's plot and life are the dominating one. By that I meant, his life set the tone and details for the original pokemon. His meeting of Green/Blue [Jap/Eng] (whom Gary compares less to than Ash does to Red), the shadowy past of Blue/Green [J/E] , and even the awesomeness of Team Rocket (Im a Giovanni fan, but most so one of Lance) would have to be the truth facts and events of the pokemon world. This does not necessarily suggest that all you completely, but it will still be drastic, change the plot and life of Ash.
Because Red is older, his adventure could occur and be completed through Kanto before Ash could have started or made a dent in his. And with his little aware outside the actually route he ever occupies, Ash wouldn't even know what's going on in Kanto. It is still possible for him to have Pikachu, since its not impossible for Oak to have found another Pikachu (maybe having found Red and his Pika to be a curious and powerful pair), and that'd align just fine. It's also possible for Ash to meet with the gym leaders, Brock and Misty, and they go on their adventures with him, as Red will need the previous gym leaders less and less as he gets further away from Kanto. Or, Ash could also just have cheesy look-a-likes to the two that then would be his companions.
Now! Considering all of that. If both their timelines could be set right and events spaced or times just right, it is possible. So. With the possible of this being possible, or more possible than people would like to admit, it would come down to; how well would Ash fit into this world? Would the darker nature of humans and pokemon cause a darker shift in Ash in Pikachu (which essentially is what would have made him into Red/Pika, but arguably not without the exact encounters and experiences of Red/Pika with all the gym leaders and other pkdex holdlers so dismiss that!)? Could that have caused Ash to keep more or less pokemon, out of fear for what could happen to them in the wild? And, how and where would he interact with Red/Pika? A Pkmn League Tournament? During an adventures? Or simply another random, friendly, but deeply meaningful encounter? It all depends...
That said. What do others now think of the idea?