Any suggested samper-frameworks for developing large orch. library using JUCE?

HISE "generates" and compiles C++ code - using the JUCE library, but Im not sure what you are trying to do here - build a sampler(with all the associated disk streaming, management issues) or build a large orchestral library, if the latter - then "just-use-HISE"

HISE can also be used as a C++ framework.

HISE is built with JUCE so you would import JUCE code into HISE not HISE code into JUCE.

However... you can also just use HISE's sampler engine as a JUCE module if you want to go in that direction. Here's the github repo (it hasn't seen an update for while though) https://github.com/christophhart/streaming_example