Three starting rebels....how many minions/rivals/nemeses can they take on in one go? How do i guage how much is too much? How many stormtroopers can I throw at them in one go?

That depends on careers, weapons and species. As for how much is too much, do some test rolls with a full squad. A full squad of 6 stormtroopers firing at someone will be 3 yellow and 2 green dice against 1 to 2 purple dice alongside 1 to 2 setback dice for cover. E11 hits for 9 damage + uncancelled successes. So a big and bulky rebel with some armour can probably last for two to three hits with some luck. A tiny and squishy one will go down in a solid hit. People usually hit in this system, at least if they have 3 in the attribute and 2 in the relevant skill.

Trial and Error has been my approach. As your groups improves their combat skills minions drop quicker and quicker. So you have to make the groups larger or include adversary opponents. They can drop them quicker but the troopers are still hazardous since they deal good damage and most players can't take more then 2 blaster shots before going down.