In ther rules set, one must have at least 5 levels in each class that he is aproved for, before level 21.
So for example sake, if approved, you could take a fighter for 15 levels, then take a rouge for 5 levels (making your character 20th level, with 15 in fighter and 5 in rouge), then you could take 5 levels of assasin, ending up with a 25th level character (15F, 5R, 5A)....Right?