Michiel:
Have you considered adding properties to the calling object and passing it in as a single parameter. Add a property to the stored form to hold the object. You can even pass the object back to the calling form with any changes in the property list. I don't like to have too many parameters passed, just because I don't like to pass dummy values when parameter 2 doesn't apply under some situations.

Calling form:
do form MyTestForm with thisform

MyTestForm.Init()
lParameter toSomeObject

thisform.CalledObject = toSomeObject

MyTestForm.Release()
Return Thisform.toSomeObject

HTH
Mike