Yes you can buy any Persona you want at any level from the compendium. I did Margaret's requests in a day by jusy buying all of them and exiting/re-entering The Velvet Room. And I have been using ztrumpeter since the first time the Compendium became available. I helps to set the money difficulty setting to "more" so you can just load up on yen for them.
Levels still should have carried over though. --- 3DS FC: 4639-9048-2382, Wii FC: 4558-6862-5396-5206, PSN: MidgardDragon DD Pawn: http://game.capcom.co.jp/DD/en/pawn_detail/?pf=PS3&gid=MidgardDragon