Objects collection

Yes, if they are in the same type hierarchy. You could also have one abstract type '_Object' as supertype for Object A and Object B and then use that abstract type in the collection.

In some exceptional circumstances it may be desirable to specify the supertype of all the types of that metatype, e.g. Object: you can do this by selecting the metatype from the pull-down list with shift held down, and that metatype will then be included in the list of types to select from. For details see Collection properties.

For an example, see in demo repository 'EAST-ADL' project and there definition of 'FeatureFlaw' object type. It has a collection property 'nonFullFilledRequirements' that refers to hierarchy (of requirements).

Yes, if they are in the same type hierarchy, e.g. Object A is supertype of B, or they have common supertype. Supertype would be then the item of the collection. For subtyping please see also discussion on subtyping after creation of Object Type.