I'm in the process this year of getting an Avant-Garde Black Metal album out with a full semi-melodic/fairly aleatroic-esq orchestra in the later half of this year. The mix for alot of that is going to be very aggressive (utilising a base setup of libraries from 8DIO, MusicalSampling, VSL, AudioOllie, Auddict, OrchestralTools, some of my own recordings etc. further augmented/obliterated with all sorts of additional harmonics forced into signals) since there's also going to be alot of content rythmically woven in from Palette FX, Angel Strings Vol 1 SFX, CAGE & CASE Bundles, Shadows etc, alongside the main layered-up orchestra from different vendors (which is very similar to what I've been doing in the last year with my orch work for fun/portfolio). So while I can try and give some advice since it's an area I'm also jumping fully into, what I might say may or may not be that useful for you.
I'd say at face-value that you'll be fine with Nucleus on Modern Mix as your foundation since you've already got it, then only start looking into layering additional libraries from different vendors if you can EQ and reverb-profile them to match up well against what you've already got. In terms of handling solo instruments from
different vendors to sit in well with your orch setup, a trick to consider would be to utilise a tool like DearVR PRO to take solo instruments you've loaded in (making sure to get them as dry as possible first), then binaurally reposition them to the exact distances you want, then apply your reverb of choice to the results afterwards to complete the blending process. That would allow you to not only take 'studio' style solo's recorded in mono/unpositioned (or even your own mono recordings) and force them to fit into sensible positions, but you can also nudge them accordingly to get them to sound clearer when you're combining both the metal-band parts and orch together.
Space in your soundstage is going to become a luxury once you've combined all your guitars/bass/drums/vocal parts of the metal band with the orchestra, so you'll also have to both from a mixing and compositional perspective potentially get ruthless with what you're going to have to cut out of sections of the tracks if you're dealing with an inaudible wall, or start looking into some extremly surgical mid/side and dynamic EQ work if you're insistent on going for a full wall-of-death result (which I'm naturally a fan of haha). Pro-Q and/or Kirchhoff will get those duties done with little issue of course.
Another thing would be to also consider what amp/cab combo's you're going to be utilising for your guitars/bass, then see how the overall tones of what you're getting sounds when paired with the orchestra (having a few other contacts is handy for this, since you can export different test-results and see what resonates with others the best). With so many options out there offering different results, having a few ready-to-go processing chains ready for you to A/B compare against can open up different perspective's on how the combination of all your results together can influence the overall atmosphere being presented. It can help to avoid having 'unfitting' processing results distract the listener too much, or if they're taking away from your vision of how the tracks should have their atmospheres presented as.
One other semi-related note: it's all good and well slapping an elaborate orchestra into these kind of projects, but just remember that a good orch setup won't make up for boring riffs dragging the rest of the work down. Make sure you've got some fundamentally solid & interesting metal going on first and you've got a 'good enough' mix of guitars/bass/drums done first before sketching down any orchestral work. If you can listen to your results without the orch being present and still think "yeah, this is pretty decent", then the confidence to start sketching the orch out should naturally come to you, since the inclusion of an orchestra in most kind of cases like this should augment a piece to bigger heights, and not act as a crutch.
If you are looking for any more/specific recommendations for getting more aggressive results in general in preperation and/or you're also looking for recommendations for aleatoric options to spice your results up, let me know and I'll throw across a list for you to have a look at
Otherwise you've already got almost everything you need already and it's just going to be down to your composing and mixing skills to get good end-results at this point.
