I personally have always liked the rhythm guitar kinda "ringing" (acoustic with 80-20 strings) so that it doesn't get into the bass/drums territory. But the way you explain your style, I suggest that you try to "interact" with your lead guitar by adding a lick or two here and there. And if you want to sound "mellow", probably "less is more" -- I wouldn't be strumming, but rather finger-picking. Classical background will sure help with that. Have fun!