So a recruiter for Newco will not look favorably upon you suing, or threatening to sue a previous employer. Who needs that hassle...

Playing Devil's advocate: No one cares about the wording, "poor performance/poor conduct". It means you screwed up and didn't meet standards. That. Is. All. You explain to Newco exactly what you did wrong and you failed as a result. This was 100% your fault. And you won't be making that mistake again. And you learned to just slow things down.

Relative to other new hires, you have the 121 training experience to succeed. OldCo already did the hard part. They brought you to the finish line. You've seen what its like, you know you can do it. You know the pace, study requirements and how to work in groups to succeed. You know what has to be dedicated to memory. And you now know your weaknesses and where to apply extra effort.

Apply to every Regional and be gratious to the one that takes you. Upgrade to Captain. Become a model employee and the rest will come, with time. You determine your success in life, not anyone else.

Now, on the other hand, if you're a jerk, ignore the above and just go sue those bastards. They probably don't know who they're messing with.