Nah, you can be sorcerer, wizard or cleric and so on. It was just I who heavily dipped on it and used outsider who's HD stacks with martial adept (Tome of Battle coreclasses) levels. It's kind same as you would pick raksasha and then take 3 sorc levels to get max of CL 10 as sorcerer which is allowed to demon prince char (or take spellcasting PrC which regs you meet with raksasha casting ability).