I applaud your thoughtfullness.
I can answer no a few different ways on this.
First, a 75 gl tank is too small for both a Zebrasoma flavescens ( Yellow Tang ) and a Butterfly ( Forcipiger spp. or Chelmon spp. ).
Second, the Yellow Tang will more than likely harass the Butterfly to no end.
Third, none of the Butterflies are considered reef safe as they will munch on polyps, feather dusters, etc.
Regards,
David Mohr