i dont know if this has been discussed before but i was wondering if Riku could use keyblades like Lion heart and Fairy harp or if he can only use Soul Eater?

Absolutely.

Riku can currently play any card Sora can, so long as particular requirements or stipulations aren't placed upon the cards (ie: Valor Form, Sora only, etc..).

Same can really be said about any card, really. The requirements and stipulations are written on the cards themselves (ie: Dark Riku cannot play friends or magic/friends, Ansem cannot play attack or equipments, Org XIII players can play Nobodies as friends, Kingdom Key can only be play'd by a light player card, etc..).

ah ok, thx for clearin that up