To submit a valid run in the All Costumes category you must first have a new save file. If you have played the game before then you will have coins and likely costumes that you can't have at the start of this speedrun. I struggled to work out how to get rid of my save data when I initially thought about doing this run but I've worked it out:

• Navigate to your "APPDATA/local" directory. This can be found at "C:\Users<YOUR ACCOUNT NAME>\AppData\Local"
• Find the folder called Montaro
• If you have launched the game before there will be a folder in here called "User Data" which contains your save files
• If you would like to keep your existing data then rename this folder to whatever you like, or if you don't care about keeping your save data then delete this folder
• Now the next time you launch your game a folder called "User Data" will be automatically recreated with blank save files within it
• Launch the game and select Costumes and you will notice that you have 0 coins and only the standard costumes

You are now ready to run Montaro All Costumes% :)

Note that this was tested on Windows 7 using the Steam version of the game so is not guaranteed to work across all systems, but message me on Discord or Steam and I'll try and help you out if you're having any issues.