Different Capacity of DG Synchronization

Yes, if they are all the same voltage, frequency, phase rotation, and phase angle (synchronized.)

See this

http://cr4.globalspec.com/thread/57066/Alternators

Assuming that your control system can handle the synchronization and maintaining the running in parallel there is a question of load sharing to keep in mind as well as the applied load. It would not be economical and fuel efficient if your applied load wasn't sufficient to keep all units at least at 75% capacity for example. Running at reduced capacities on all the units would be fuel inefficient. It would be better to remove a unit or two to maintain the running units at or near their full loads for best efficiency and keep a unit warmed up and ready to start and parallel for peak periods.

Yes U can, but all the generators must be very healthy and able to speak the same language of equal frequency, equal voltage, equal PF and same phase sequence.

The issue of utilization and fuel saving depend on the healthy nature of the generators and how the automatic controlling function responds to increase and decrease load demand. Usually the controller will start up and switch in more generators during increase load demand. During off pick periods, the controller shots down some of the generators and place them on stand by.

The above can also be done manually.

DICKSON