1. As for linking with assimp: I'd need more details to help with this.
You should get a regular dynamic library by compiling libassimp, with which you can link as usual. You can find many examples e.g. in CGE (we link to libraries like OpenAL, FMOD, VorbisFile, LibPng) or FPC how one can link to dynamic or static libraries.
Note: This topic can easily get longer :), so it may make sense to move the talk to our forum ( https://forum.castle-engine.io/ ).
2. As for other libraries:
Well, Castle Game Engine could actually be used as a model-reading library :) One can use just `LoadNode` routine from `X3DLoad` unit to load all supported model formats ( https://castle-engine.io/creating_data_model_formats.php ). And then convert the resulting graph of nodes to whatever you need.