I have often been accused of making encounters too easy so I started dialling it back. Unfortunately our most recent session was far too easy. I've got 8 characters to work with now having just acquired two Jedi. I'm worried about putting in too many rival and nemesis level npcs incase we go back to being too hard.
Any advice?
8 PCs can soak up a lot of damage and put out a lot of damage, I wouldn't worry. Ultimately just sit down in between sessions and do a test scenario or two and roll it yourself and see how it plays out, that's the best way to check things.
You have too many player characters to deal with in order to run a precisely balanced game; whatever that is since FFG SW is narrative and doesn't use encounter CR levels. I usually run 4 - 5 players but if I doubled the number of opponents a module suggests to accommodate 8, that exponentially increases the likelihood of one of the PCs being immediately removed from combat or killed.
My suggestion would be to keep your encounters as they are but rely less on combat and more on other opportunities for role-play and skill or talent use.
Don't forget to run your enemies in waves. If 4 rivals start dropping fast, 4 more run in the door. Supported by minions, if necessary. Scale up on the fly.
Try to split the party, if you have 8 PCs. That way you can run multiple combats simultaneously and balance the threat to the party currently engaged.