Eh, well, I guess if you're not sure you get all the vitamins, etc. you need, you could take a multivitamin. You don't need anything special though. The growth speed varies according to corpulence (thinner = less breasts, in all likelihood), age and genetic factors, but I think it's safe to say that after one year or less (assuming you have a decent dose, unlike me) your breasts should be very noticeable and maybe at 75 % of their final size. After two years I think it's safe to assume your breasts will be very near to completion. I think they'll then grow very slowly until they finally reach their final state, after a total of, what, 3-5 years? But it can divert from that a lot.
As for cancer... I think the best answer is "because they're ignorant". IF you somehow get a specific kind of cancer, THEN hormones can encourage its growth. But on HRT you're going to be followed by a professional who'll make sure that doesn't happen.
Oh yeah, and your voice... Once you train yourself to talk like a woman (which 99 % trans women need to do, because sadly, voice isn't changed in any significant way by HRT, unlike in FTMs) it's no big stretch to train yourself to sing as such as well. Of course soprano is quite a stretch, but alto is probably not out of reach if you work hard. And if not, well, contralto, which as I recall is just a barytone who sounds female, is absolutely, completely doable.