The Senario: While exploring the bajorian wormhole the federation encounters the empire.
Rules: Canon material only, no temporal interferance.
Timeline: Hight of Empire, just before the battlend of Yavin 4.
Deep Space 9 just before the first encounter with the Dominion.
Additionally: The Q set it up so they will no get involved, expect to annoy significant characters. The rebellion still exists but they are severely weakened as the empire will retask the death star and send a fleet of star destroyers instead. The klingons will quickly join the federation, the Romulus will but it will take time. The Cardassians will ally with the empire.
Well with the response I expected you to have the defiant cloak and take out the thermal port on the death star. It certainly was creative.
As for the tholians it seems unlikely they would help they are extreme isolationist after all. Really they would only be likly to intervien for temporal tech or a direct attack on them.
Use of the travler is clever but unlikly to work too well. As far as we know he can only boost one ship, and a planet killer may be too large. Xindi or neuronium planet killer?
Also planetary battles would be realisicly be unlikly by st forces as by TOS they can fire highly accurate phasers on a planet even on stun. Though with the exception of the borg(and it seems unlikely they would involve them), sw ground forces could take out really any st ground force.