For starting mockups, The Orchestra Complete 2 from SonuScore is amazing.
Here's an example of ideas using just a single note melody going thru some Logic Pro X scripts I wrote that generates chord formulas from a select scale/key and where I can also set the note length and octave range to use as well.
This video shows just one instance of The Orchestra and only one MIDI Region (the simple melody), but you can load as many as your computer can handle.
I just kept this video very simple.
I'm also using the supplied presets, remember you can create your own presets within The Orchestra and modify existing presets.