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You can connect more than one device to an account but they have to be different types of fitbit and they have to be your fitbits - you can't connect a fitbit used by a different person as this would require a separate account to record their profile details (age, height, weight etc).
Yes, you need to set up a family account in order to set up the Ace. See: https://help.fitbit.com/articles/en_US/Help_article/2346/