If the xformer is intended for Flyback power supply, it needs to store as much energy as you need.
If it is the usual xformer, it needs to store as less energy as possible i.e. ideally transfer all the energy to output.
2)what happens if i short one coil of the transformer in power supply?
If it is the coil in the same side as the supply, nothing happens. If it is in the secondary side, the current that will flow will be very big. How much can the xformer withstand, depends on the short circuit test you previously did on the xformer for its characterization.