Very new to AudioJungle, so please excuse the noob question: 
Can I upload versions of a track separately and then later on create a music pack including the main track and its versions? Or can I only include entirely different tracks in a music pack?
Looking forward to meet everyone here btw! 
Hi MagicRocket, welcome to AudioJungle 
You cannot upload versions of a track separately and then later on create a music pack with them.
For Music Packs, only items that are already accepted can be included in the upload. You can see more detailed instruction on how to submit a Music Pack here
As for separate versions of track, you can upload one Remix version separately only if it is distinctly different in terms of arrangement, style, tempo, etc. Otherwise if the other versions are similar, being just shorter, or mild edits of the main track, you need to follow the instructions for including variations with a submission.
Another good way to see various acceptable ways to do upload packs is to look at some of the items in the Music Pack section
Hope that helps you.
Got it, thanks for the detailed reply, ADG3studios!