I have used that technique, it is very easy to get the graft to take but long term survivability in my climate is low and I get flowering within the first year. A 12-18 inch tall seedling is much to young to hold fruit.
See this thread
http://tropicalfruitforum.com/index.php?topic=16549.0Since you are in a marginal climate, I recommend creating a double rootstock seedling tree without grafting on mature scion wood so that the tree will grow vegetatively until it is the proper size for fruiting. Much energy is wasted on flowering in young trees. Trust me, I speak from experience.
Simon