RE: regarding custom schema

1. Second choice of making new objectClass STRUCTURAL is fine for me ,

but will there be any restrictions for new STRUCTURAL object classes to be created from any parent (for instance, organizationalUnit)

Only that they be STRUCTURAL.

And from this, new STRUCTURAL class, if I derive one more hierarchy down , will there be any restrictions for that grand child object class.

None beyond what apply to all object classes. Proprietary/custom schema is treated no differently than standard schema.

2. Can I know why is that restriction that AUXILIARY cannot be derived

from STRUCTURAL?

Because it makes no sense. Look-up the meanings of AUXILIARY and STRUCTURAL.