To answer your question . . . yes, the metal halide will provide all the light that your corals will need.
On another note, the correct spectrum for corals is anything above 6500K . . . it makes no difference really to the coral what color the light is. The only difference is really for the viewer. The higher the color temp, the bluer the light, the deeper the light will penetrate, and the more your corals will glow. But for example, 6500K has been shown to produce the most growth rates in sps corals, but it is a yellow color which most people find unattractive. I would not buy the 6500K T5's.
I would do one of the following:
A.) Return the T5 setup and just go with the 20K MH
B.) Retrurn the 20K MH bulb and buy a 10K MH and use T5 actinic supplements (my personal vote goes here)