Writing the orchestra for a symphonic metal can be tricky.
Sometimes, I think that there is nothing needed to add since there are drums, bass and several guitar layers.
Well, and vocals and choirs, of course...
But to get that monstrous wall of sound, you need even more!
And you have to be careful to not overwhelm some specific frequency ranges too much.
Of course, you should think about in which octave you should write another track to not clash with other tracks too much so you can choose the instruments that naturally sound good in higher octaves.
Bells, ultra-high noted strings, flutes, chimes etc.
But what about the piano?
Piano can be also tricky.
For piano, I sometimes copy the piano part and transpose it one octave higher and mix it with the original lower note piano track.
This can make wonders in terms of clarity and still being fat.
What about strings?
Sometimes, guitars and strings sound just great in the same octave range and you simply don't want to transpose the strings one octave higher.
For example, you can use staccato strings over the dense chugging guitars in the same octave range.
Because staccato just emphasizes the beats, its dynamic, adds movement and is not clashing with other tracks all the time so it's better than just keeping long legato strings over the guitars.
If it sounds good, it IS good :)
Even it's in the same octave range or in an almost the same octave range.
May the muse be with you!