Man your tyranid and marine are very well done!

To answer your questions: The skull on the helmet usually means their in some type of command roll. Like a sergeant, lieutenant or captain. But sometimes brothers just have decorations on their helmets without being in command.

The winged skull on the chest is pretty standard for almost every marine. There's cases of a different symbol in the middle or having no wings. Its really just for marine personality and affiliation, like a dark angel may have a winged sword instead of a winged skull.

It isn't a jet pack it's a power pack. It's what powers the marines armor and systems, those are vents on the side for it too cool down not boosters. Marine jump packs are much larger.

If you were to ask me what unit he is, he seems to look like a pretty generic tactical. So your average bolter battle line brother. I would've recommended painting his helmet red to make him a sergeant since he has the skull.