Mystery skins contain legendary, ultimate skins and legacy skins and challanger Ahri falls under legacy, so you should be able to obtain her. As for the second question, you can only obtain skins for champions you have yes.
If you have any more questions regarding skins you can obtain in mystery gifts, you can only get skins worth 520 RP or more. All skins EXCEPT achievement skins, for example ; Collector's Edition skins, PAX skins, Rusty Blitzcrank, Urf the Manatee, Championship Riven and Riot Squad Singed.