The inertia constant has no effect in load flow calculation.
Normally you select a large generator as swing bus. However, any bus can be selected as swing, there is no need for a generator at the swing bus.
The most important is that the swing bus is electrically strong and is able to absorb any active power imbalance created when tripping generators, loads, HVDC lines, etc or after scaling total load/generation in the whole network or a subsystem.
A swing bus at high voltage level (e.g. 400 or 500 kV) is stronger than a swing bus at generator level (15-25 kV).
When performing voltage stability studies it is also good to locate the swing bus outside of the studied area, since the swing bus has no reactive power limits.