You can config a domain-level setting in domain profile page (Tab: Throttling). Per-user throttling is available in user profile page too (Tab: Throttling), it's allowed to override domain-level setting (Please see screenshots in above link).

Yes, you're right.

Settings in config file (e.g. /etc/policyd.conf) will be used if neither domain-level setting nor per-user setting is available.

Not achieved in iRedAdmin-Pro-1.3.0 yet.
According to policyd doc (http://policyd.sourceforge.net/readme.html), it's possible to set limit like this:

INSERT INTO throttle_rcpt (_rcpt,_count_max,_time_limit, _date)
 VALUES ('@hotmail.com', # recipient address
          100,               # maximum messages per time unit
          86400,             # time unit in seconds (1 day)
          UNIX_TIMESTAMP()); # current time

1. If applied throttling on domain, it will be a domain level or user level?
Domain level =  if one user within the domain used up the quota, no other domain user can send out email within a period.
User level = the throttling is a default setting of every user within the domain.  It can be overwrite by the user's throttling.

2. If few user within the domain will set to unlimited, the other will use a standard throttling (e.g. 20M per hour).  Should I put the throttling on domain level, and set the unimited to the specific user?

3. WIll the domain or user throttling overwrite the configuration setting?

4. Can it limit outgoing domain?  e.g. To hotmail limit to 1000 mail per hour.