There's a definite drop in quality when the mono elements are dropped in. For those, you may want to consider using the 5.1 mix as a jumping-off point, getting rid of the center channel and downmixing everything else to stereo, and then mix the mono version in *over* that, so the quality difference isn't that noticeable. Unless that's completely ludicrous, which, given my limited experience in such things, is entirely possible.