Do I need to say I'm going for Legacy, or Any%, or can I do the Legacy one, and upload the run to both?

general rule of thumb is only submit to one board, technically any run could be submitted to any%, if you run legacy, just submit to legacy and it should be all good :)

One run can be used for one leaderboard, this is to prevent crossposting and just general weirdness that can cause and for ease of moderation.