For my 02' caddy deville, i'm getting a pair of Rainbow CMX Kick 6 1/2 speakers to replace my stock system. (Bose). The back seat speaker sizes in the deville are 5 1/4. i don't think i cmx kicks come in this size, so what would you reccommened for speakers for the back seats? is it alright to mix diff. speakers for the front and rear speakers? thanks.