Yes, you can combine Rebuke and Steadfastness. In general, you can only use 1 tactic per combat, but there is no limit to the bonuses you can use.
When you exhaust an active power, it becomes suppressed:
Rule 6.2, page 6 wrote:
In the event that a power is exhausted while it is currently active, that power becomes suppressed. It counts as exhausted and none of its effects can be used (including active effects), but when the power is refreshed it will automatically become active again (on the same target, if any).