The tangs should get along just fine, esp. with 175 gals to share I'd expect some 'picking', but not likely bad enough to be a detriment.

The problems with mixing tangs usually come from A) too small of quarters, and B) mixing those with like body types. The yellow tangs have that high finned shape, while the powder blue is more of the teardrop shape -- they may compete a bit for algae around the tank, but they shouldn't fight trying to be the 'alpha' of their kind

-Justin