Starting in medias res

I'm currently working on a Pokémon fanfic and I had the thought today to start it in medias res (i.e. start with a scene, then flashback and progress to the point that it started and onward). I think it would be great with a chapter I have written that's further along, but I have a few questions:

1. Is there a such thing as being too far ahead for medias res to apply? If I wait too long to get back to that point, will people forget about it?

2. Once I get to that point, do I repeat the scene in some way or skip to what would happen next?

3. Can you give away too much in the opening scene that it becomes too spoiler-ish? (Not sure how else to word it)

I've personally never done this, but I'll give my two cents.

1) Yes. If you take too long on the flashback, people will forget the opening scene. So make sure that the opening scene is something you can remember, and the flashback isn't too long.

2) You want just enough for the reader to be able to tell that it's the opening scene. Too much detail will make the scene bland and boring, as they've already read through this, yet skip over it too much, and they completely miss the reference. It's a balance you have to achieve.

3) Yes, you can. You don't want the opening scene to be too long, or else there will be too much to cover as you recap. You want just enough to be memorable, and that's pretty much the key.

Another thing that will help is flashing back while the character is doing something important. Say we have this girl named Kelly. Well, Kelly is saying goodbye to her best friend Darcy, who is moving away. You can't go back as they're saying goodbye, as it's not that memorable. Instead, go back when Kelly is waving goodbye as the moving truck pulls away. That way, an action is left in the readers' minds instead of a line of dialogue. That way, you can go back to explaining how the thought of saying goodbye nearly tore their friendship apart, and come back to Kelly waving goodbye without losing the readers.

I remember that when I was young...er I was a huge fan of a fantasy pre-teen book series that always started in media res in the action of the final battle scene, and then would say "oh, but here's how it all happened." I really hated that. It was like it was baiting me to read the rest of the book.

Starting in media res can be totally fine, but don't do what I just said! XD Sometimes in a literary work, you can start in media res without a flashback, and the action shows you details of the protagonist's life or other important things without telling them to the reader. I prefer that kind of in media res.