Jenga

I think IImarcus meant that the starter pokemon data in it is scripted.
Then, you've to load the script of the pokemon contained in those pokeballs and change them from there.
(A starter pokemon editor for that would be nice idea too...)

"By the way, I have a question about editing the starters in Gen I games.
CoolBoyMan's Rom Map says it is from 3A1E3-3A1EB. Well, I found the pokemon hex there, changed to the ones I wanted, but then only the rival's pokemon change, mine remains the same."

I can't help you any better than this:
As the offset is between 3AE13 and 3AE1B, there are only 11 (10 if the last one is FF/the first one 00) bytes in that "area". There must be at least 12 bytes of data for storing both the pokemon you and your rival receive (one for level and the other one for the pokemon number).
I think you should ask Coolboyman if there is a mistake in that Rom Map or if the data, for the pokemon you receive, is somewhere else.

I hope that I made sense :)