I'm not 100% sure what you're asking, but if you want to know how to get all the weapons at the very start of the game: It's impossible. To my knowledge, there's no way to start a file with all the Legecy weapons. There may be a gameshark code, but I wouldn't know.
Howevere, you MIGHT just be confused about it. When you start a new file after beating the game, your weapons will retain their levels from your previous file, along with their ley seeds. And you'll get a new weapon as well. The twin blades.