What units and structures in Starcraft can be repaired? Can allied units from other races be repaired?
I just found this which answers all questions!
SCVs can repair Terran buildings or mechanical units (including other SCVs) under attack which is a key part of defending early game pushes.
Basically put: SCVs can repair any Terran structures or any mechanical units. "any" isn't in the original quote but I want to put emphasis on the fact it does not narrow down mechanical units to a specific race.
Of course they can only ever repair allied units. As a side note, Zerg does not have any mechanical units and if he Neural Parasite'd one the unit would still be Terran or Protoss.
SCVs can repair any mechanical units, Protoss or Terran or Zerg(if they manage to build any, eg via NPing a worker) of any of their allies. They can only heal health, and not shields, however.