This question already has an answer here:
- What can SCVs Repair? 2 answers
- In a 2vs2, 3vs3, or 4vs4 matchup
- In SC2 Wings of Liberty
- Repair structures, not units.
SCVs can repair anything that is deemed 'mechanical', Terran Buildings are mechanical, Protoss buildings are not.
No you can't but still there is great synergy between Protos and Terran.
SCVs can repair the HP (not shield) of friendly Protos units except Zealot, High Templar and Dark Templar. To heal these biological units, you can use Medivacs.
SCVs cannot repair non-terran structures. They can however be used to repair a variety of Protoss units (Hit Points only, NOT shields):
For completeness, here are the list of Terran units that an SCV can repair too:
SCVs can not repair anything from Zerg, sorry (although if a zerg player uses neural parasite on a mechanical unit, it can be repaired by an ally's SCV or an SCV that is also NP controlled).