My advice is have someone tell this story about the character, and no-one is sure if it is true or not, and not actually clarify whether it is true or not. Add a bit more intrigue to your character and give your readers a choice; do they accept the commonly believed origins or do they come to their own conclusions?
I think that it's important to think through why you want a prologue. I'm not saying this as an anti-prologue person, though I am mostly that. But because I think that the pro-prologue would also agree that a prologue needs some reason for existence in prologue form.
Well, that depends on what happens after the skirmish. Or do you mean the prologue would just be Mystery People finding a Mystery Baby? That's not any better, really. It's still a scene that would mean nothing to the reader. I think you need to question what the point of having this or any prologue actually is.
I agree with @matwoolf. I think (from what you've given us) that you're worrying too much before you even start. Write your story however you want to write it, starting however you want it to begin. The further you go, the more your questions will answer themselves. Remember: nothing you write in a first draft is set in stone. You can change ANY of it that doesn't work later on. You won't know what's going to work and what isn't till you get it written. Don't worry about clichés either. The reason something becomes a cliché is because it gets used a lot. The reason it gets used a lot is because lots of people like it! So if you write it well, make these characters come alive as people, not character lists, it doesn't really matter if that kind of thing has been done before. Folks will still enjoy it. As to the prologue question....unlike some others on this thread, I do like prologues if they're done well and serve a purpose. Their purpose is not simply to start a story. That can be done in Chapter One just as well. What a prologue does is let the reader in on something they need to know or think about before the story starts. So if it's important, before the start of the regular part of the story, for the reader to know that this baby was rescued in this odd way, then you can probably use a prologue to open the tale. Just make it as much fun and as interesting to read as the rest of the story. However, don't fall into the trap of thinking that a fight is automatically interesting and we're going to be on the edge of our seats if you start with one. It's not interesting at all, unless we a) care about the combatants, b) understand the stakes or c) there is something really unusual about the fight itself. If it's just guys whom we don't know killing each other, whack by whack, we are not going to become engaged here. We'll start paging forward to find out what the the heck the point of this fight is. Or we'll just stop reading. However, don't fret about any of this just now. If you want to write a fight scene to open, go ahead and do it. And then keep going. Keep going till your story is finished. THEN you take a look at it, and see what you've got, decide where changes need to be made, etc. Mat is right. Don't worry if you're doing it right. Just get it written. ................ Just as an aside, regarding opening with a fight scene. I'm thinking in terms of movies here. I remember sitting down to watch the first Daniel Craig outing as James Bond (I've never been much of a Bond fan, but I did like Daniel Craig, after seeing him in a couple of other productions.) And wham bang, it starts with a fight, between the Bond character and some other guy we don't know anything about. At first I was prepared to be bored, but then, after a minute or two, it hit me ...hey, wow, this guy (Bond) never gives up. I mean, Bond just kept going, no matter what the other guy did. Bond's relentless pursuit provided the focus for that opener, and what an opener it was. Fantastic character development. Of course the fight itself and that particular criminal didn't matter to the story. But it was very important to know, before the story starts, that Bond DOES. NOT. GIVE. UP. till he lands his quarry. In a book, that would have been a prologue, because it doesn't actually fit with the rest of the story, as regards the plotline, but it was necessary for the viewers to recognise that Bond trait ahead of time. He's not superhuman, but he does not give up. That's how he wins. Excellent employment of a fight scene in a movie prologue, in my opinion.
