Composite type: Primary Key and validation

Hello guys,
I've a couple of questions about composite type.
Suppose this composite type:
CREATE TYPE my_type AS (
user_ts_start My_start_timestamp,
user_ts_end My_end_timestamp,
db_ts_start My_start_timestamp,
db_ts_end My_end_timestamp,
audit_record jsonb
);
My_start_timestamp is a domain of timestamp with default as now().
My_end_timestamp is a domain of timestamp with default as infinite

1. May I use user_ts_start and/or db_ts_start has part of Primary Key of
a table that contains a field of my_type?
2. to add an overall check constraint on the entire composite type,
could be a valid approach to create a domain based on my_type and add a
custom function to validate it? (check_my_type(VALUE)). In this way I've a
dominan of composite type that contain others domain... what do you think?

thank's in advance

--
Domenico L.

per stupire mezz'ora basta un libro di storia,
io cercai di imparare la Treccani a memoria... [F.d.A.]