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.
Star Trek fo sho. Shield technology is far better, even small shuttle crafts have basic shields and don't need the big shield generator equipment.