Can I use a field set by another rule in the criteria for a rule?

A rule’s criteria should not be dependent on another rule’s action, as the system will not be able to execute either rule. For example, if your system has a rule which sets the value of a field, do not use that field in the criteria for another rule.

Will a rule be triggered when an aggregate field is updated?

Case IQ will not trigger a rule when the system updates the value of an aggregate field on the case form. This means that, if you have configured rules where the "Rule Type" is "Record Event" and the "Record Event" is "Case Updated", the rule will not be triggered when the system calculates and updates an aggregate field. See the Aggregate Fields on Case Form tab on the Edit a Standard Record Type article for details on aggregate fields.