Latin vocals + symphony orchestra + rock, and I mean Latin not latin-american.

All I know of is ERA. Are there other such groups?

I'd suggest you Therion, but it is rather gothic metal. You really want the songs in Latin? I'm not sure, but some their album contains few of such pieces..

For similar artists search I use cdnow.com. If you're not sure, you can listen to the samples.

well a second best option would be a symphony/orchestra + rock, but no singing at all (or very very little)

thanx for the advice mykhal!

Maybe Gregorian?
Masters of Chant Chapter II is their best album so far