Scopas would not be much different in care than a Yellow tang really except they get a bit larger. A 75 gal
may not be great long term but it would do as a start and give that excuse you needed to justify an upgrade. Zebrasoma sp
tangs in general do fairly well but careful attention should be paid to diet and water quality.
You might have a little aggressive tail slapping with the bi-color. It will depend on the realitive size of each so be sure the Scopas is at least a similar (or larger) size.