From: Chicago, Illinois
They will merge automatically, when there are fragments from the same unit in the hex. Maybe there can be problem if some of them in Strategic mode, but I am sure move, and combat do not stop merging.
Unless some of them upgraded to new devices, but I am not sure if this is possible for fragment.
Not sure about that. You can have a parent and sub-unit in the same hex but you still have to manually unite them. You won't be able to if they're equipped differently, though.
Fragments and sub-units are different, though.
Sub-units are entire units that can be "Rebuilt" into a larger unit using the "REBUILD" button. This is not automatic, as you say.
FRAGMENTS are just that; pieces of whole units, like "3rd Battallion/1". Fragments do auto-combine when together, and won't take replacements
Two totally different types of units