Can we initially start domian with a single node or a single server and then later on add more nodes/servers to the domain and create a grid?
Rupesh : Above statement is correct, you can create a domain on single server and then later join servers to domain.
GRID requires atleast 2 servers which should be part of same domain. Once you create GRID, assign integration service to run on GRID.
For HA/GRID you will find documentation in Administratior Guide --> High Availability chapter.
Hope this helps.