If by story/legacy you mean something that is a roleplaying game (a game in which players assume the roles of characters in a fictional setting) then you're fine. If you mean like an actual
legacy legacy, than you might want to check out the sims 3 stories forum.To answer your second question, if this is a true roleplay, then there will be other people playing, but they will be playing as their own characters and it's up to the creator(s) of the roleplay to monitor what happens. I'm sorry I can't be of more assistance, but if you could explain what you mean by "story/legacy" then I could probably help you more.
EDIT: As for being allowed to do it, as long as you're not breaking any site rules (you might want to read them, just to make sure) you'll be fine.
THE CAT HAIR WIZARD