IMO a bad place... when you change the object/type/method later you have to change the form class. if you use one time array class and one time classmethods you need...
a) 2 base classes
b) set 1 time a new hydrator/object....

IMO this should be an explicit decision. If you use objects in a form it should be required to set object in glue code and not in a form class itself...

maybe the hydrator should be set by the data-object itself? Object should now best how it should be handled... Is there already an interface for like for filters? Is this always possible? Maybe a hydrator parameter on bind/setObject makes sense?