Are spoiler tags the new trend or something?
Welcome to FF&W as well as the world of Fanfiction, dbcification! It's an exciting world, writing is. Length is not a concern. If people criticize you for length, it would be about how you rushed this, or it didn't flow like this, or this wasn't shown to be important enough, or anything else. Overall, it shouldn't matter. Long or short, as long as it's good writing, it's fine. People do not get turned off by long stories, trust me. I mean people read books all the time. I read the last Harry Potter book the day it came out because I was so excited, despite it breaking 700 pages. It did take me all day to finish it, but that's okay. It was a really good read and a satisfying ending. The same applies for fanfiction. Write as much as you need to get your story across, that's all that really matters.
The protagonist leaving isn't a prologue, I'll tell you that right now. A prologue would be some big event that sets your story into motion, like 9/11 caused the war on terror. Yeah, that's not exactly a story example but you get the point. Preferably, you should write in chapters. A lot of authors have acts, but that's just to separate the chapters. Not many release their story in acts. Use chapters. It's neater and easier to track, posting each chapter one at a time.
So now, onto the egg and Sneasel. Yes, they are known to be seriously vicious, but that doesn't mean there aren't exceptions. I don't see why that would be so important to Professor Elm; it's just one Pokemon. It's temperament is a bit more mild, but overall it's still a Sneasel. It doesn't have three arms or an extra head, so why he'd be interested is beyond me. Professor Elm studies eggs, right? It would more canon if Professor Elm wanted to study the aging of a Pokemon hatched from an egg in captivity versus one in the wild, so he could ask Ethan about that and his help in research that way. In this case, you can mention how different its nature is from other Sneasel because of the way it was raised, you might mention egg moves, there are a lot of things you could do in that case. It's far more interesting rather than, "This Sneasel wouldn't hurt a fly. I think I want to study why!" If he studied people like that, he'd study everyone. He'd wonder why some people like this type of music and not this, because that's the standard kind. He'd survey what their favorite hairstyle was, and he'd be running around the world with a lot of questions. He'd wonder why some people can solve math problems, whereas others can't without a calculator.
Ironically, Ash is like that. I mean the way you're describing the character, since you want to use his mindset. AKA, being a complete doofus, can't tie his own shoes without yelling Thunderbolt, can't put 2 and 2 together (literally), etc. He also loses his temper easily, like when he met Paul in Sinnoh. I swear every time they met, Ash tried to act cool and Paul called him a loser and voila, Pokemon battle! The same goes with Trip, because that prick calls Ash a hick all the time. Come on Trip, get some new insults. Red neck jokes get old. Anyway, your character wants revenge? That's a different meal. Revenge is a meal best served cold. That's a big tone, in my opinion. The antagonist would have to do something more than just piss him off. It would be interesting to see his demeanor change as he continually meets his rival and his Sneasel feeds off that dark personality. And then they keep adding their negative influence, so you get some really messed up character. And that's freaking awesome. Now that does complicate things since you want that goody goody stuff in him. I mean, the only thing you can really do is change him. Don't make him be like Ash, a snot-nosed, selfish kid, and make him really chill or something. Either way, you'll figure something out.
Oh, one last topic: Eggs. Nah, that's not a big deal. Pokemon are practically worshipped in their world, so receiving a Pokemon egg is like catching one without the effort. The problem is how they receive the egg, but you won't reveal the plot so there's nothing else I can say.
Yeah, I finished! I'm so happy. Well, now I wasted about 10 minutes typing and doing nothing but help other people, when I should study. But studying is really boring and I prefer writing and reviewing and that stuff. Oh well. Big woop. Good luck with your story and I'll be looking forward to reading it!