I know that it does state somewhere that the number of cards supplied does not mean that you cannot field more, unless the rules say otherwise. It may have been stipulated in the Space Marine FAQs published in White Dwarf back in the 1990s.
In terms of the restrictions which do apply to choosing your army, see pages 13-14 of the Space Marine main rulebook, as well as any supplementary rules published for your own army. In the case of Chaos, this would involve looking at the Renegades rulebook.
_________________
SoƱando con una playa donde brilla el sol, un arco iris ilumina el cielo, y el mar espejea iridescentemente.