If I have a Rainbow Energy attached to Simisage, would its "Fire's Power" attack put a Burned Special Condition on the Defending Pokemon?
Yes, Rainbow Energy counts as all types of energy when in play, so it would count as Fire for that purpose.
Source: Emerging Powers FAQ (2011-09-01), PUI Rules Team (2011-09-01)

Relevant cards

Note: All applicable cards may not be listed below.
BW2_2