If you mess up, you just don't save the game. That way you can start over.
Anyhow, I'd say you should do a catch of a pokemon or a breeding project in HG rather than Platinum, because they don't do frame advances the same. It's better if you just start with HG. I have some guides on my thread for capturing standing legendaries. If your latios isn't roaming, these guides will work for you. If it's roaming, do another capture (or several) on HG until you know what RNG steps you must do to get pretty much all pokemon.
Good luck. I'm signing off since it's so late here.