User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-24916326-20140209120003/@comment-1359322-20140209142623

Not really, Evil Pieces are meant to enhanced the original ability of the the reincarnated servant. Also using the wrong Evil Pieces on the wrong servant usually leads to the servants ability being reduced or affected. Should Rias use a speed-based Knight Evil Piece, it would reduce her natural power and defence ability, since Knight Evil Piece reduces power and defence in exchange for speed.

Take Rossweisse as an example, her supposed position as a servant would be a Bishop, but since Rias used up both her Bishop Pieces, she used the Rook Piece on Ross. As a result, Ross and the Rook's Piece incompatibility resulted in her losing to Heracles. So Ross actually had went back to Valhala to train herself to fit more into Rook Category, but then again that is also because of the Norse's advance magic system.