After questioning Dr. Zee from USDA Hawaii on the Fi Tsz Siu lychee grafting, his responses confirmed the infos posted on the link on a previous reply.
According to Dr. Zee, these are keys factors in lychee grafting:
1. Rootstock must be growing vigorously (could be seedling, air layered, grafted)
2. Rootstock must be at least 1 yr old, half inch size is best
3. Rootstock could be Hak Ip, Mauritius, or Brewster