Hi Daniel, thanks for your reply.
I figured it out, I had to UnwrapElement(family_inst) and then for every element in family_inst , had to convert ToDsType(), now all family instances are of type Revit.DB.Element.
Here it is:
import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import *
clr.AddReference('RevitNodes')
import Revit
clr.ImportExtensions(Revit.Elements)
clr.AddReference('RevitServices')
import RevitServices
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager
doc=DocumentManager.Instance.CurrentDBDocument
for i in family_inst: #convert familyInstances to revit.db.elements
new_types.append(i.ToDSType(False))
OUT=new_types