Ehh a little lower would be ok. I personally want there to be some overlapping frequencies in my different tracks, as there are plenty of other ways to distinguish instruments apart from one another.... such as stereo panning, distance (verb), volume, dynamic range (amt of compression), and not to mention the rhythms and pitches being played. The composition shouldnt have too much competition among different instruments in the same frequency ranges to begin with.
Still, I understand your goal of keeping the low end mud-free. Some blending makes things, erm, blend. I would try High Passing (low cutting) around 400 like you said, though it depends on the instrument and what it's playing. Try using a HPF with less steep of a rolloff, so those frequencies arent entirely cut right away, but gradually decrease in volume.. Or just scoop the lows to taste with a parametric equalizer... use your ears!