My crew has just hit 250XPish earned. I'm seriously needing Adversary ranks for boss types. I used it for the first time about 2 weeks ago and I was shocked at how bad the Red dice were for the players.
So... Wing it... You shouldn't need too much. There are a lot of other balance issues that you will have to figure out in the fly. Like what is a fair fight? How come the players get 6 turns before your boss gets one? That stuff causes way more issues.
Read my thread on my players invading the Jedi Temple. It explains what it was like for the players, and at the end you can see my thinking on balancing up an escape space battle.
Regarding the slots/round issue, the last bit in the GM kit says about giving true nemeses an extra turn each round in combat. Last night I had my players fight a nemesis 4 on 1, but to even it out he got 2 shots per round. Made for a pretty even matchup, and more importantly a fun encounter.
Your Nemesis lasts a whole round? Interesting.
The last fight I ran, had 4 Nemesi with Cortosis Armor (No pierce)... That was a fair fight for my guys. (I measure success by how much I make them cry.)