Thanks @benvanstaveren,
RAM is abundant on these servers and we have little other use for it, so what will likely happen is each box will get a single VM (likely go with ESXi now instead of bare metal) for ES with something like 100GB, 64 for ES and the rest for activities.
It’s been a long time since I built anything GL/logging-related, but I’m guessing the best approach is design + build the ES cluster, then get MongoDB running, then finally install GL?