Hi Henry,
Thanks for the input.
Here is what we did to solve the issue:
Since all the message types had somewhat similar node elements in common with different orders with no specific sequence, we designed a single common schema definiton with "Group Order Type" set to "All" instead of "Sequence".
Surely that was not by itself enough, we had to set the "minOccurs" property of the uncommon child record elements to 0. By this way, we were able to produce a common schema for all messages. Also the need for a custom receive pipeline was eliminated, the default xmlreceive pipeline was just sufficient.
In fact this workaround was suitable for our situtation where incoming message types shared a common backbone and things are not so complex.
Thanks again for input and hope this helps others.
Emre.