You and your co-workers likely have a FLSA claim. Often this can be resolved with a demand letter to the employer with the threat of litigation. Most FLSA attorneys will work on a contingency and get paid by the employer.
It is quite likely that your employer has violated the Fair Labor Standards Act. Generally speaking, your employer must pay you overtime for any week where you worked more than 40 hours. You will need to do some further calculations but this is certainly worth investigating.
Generally speaking, non-compete agreements are enforceable in Pennsylvania. That being said whether a restrictive covenant will be enforced against a particular employee requires an examinaiton of all of the surrounding facts. Given that you would be joining a group of former employees I would fully expect your employer to pursue you.
Generally speaking, non-competes are enforceable, but they are not favored by the courts. It sounds from your question that you have some extenuating circumstances that could be used to defeat your employer's plan to prevent you from working. You should consult with an experienced attorney who should be able to develop a winning strategy.
The important piece of missing information is: what was the "customer related issue" that you discussed with your Manager. If your complaint about the customer could be construed as "protected activity" then it is possible that your Manager retaliated against you.
I assume that you are an at-will employee. That being the case, you will need to demonstrate that you have been discriminated against. Are you in any protected class: age, gender, race etc. ? And assuming that you were in a protected class, do you believe that your termination was motivated by discrimination? If you have been offered any severance and given a release, you should have an employment attorney review it for you.
The short answer is that an employer MAY change an employee's compensation prospectively (i.e. into the future). The employer may NOT make that change retrospectively. You may have some argument that the change has impacted commissions that are already owed but it is difficult to tell from your question.
Having been in this situation in many cases, I would say that if you do not voluntarily speak with the attorney, then you will receive a subpoena to testify. Alas, this is our legal system, not unlike jury duty.
An employee is never required to negotiate for his/her pay. All wages that are due to you should be paid as part of the regular payroll. If you are asked to sign a Release then it should be offered along with additional consideration (i.e. more money). If you decline to sign it you get your wages regardless.
You are obviously aware of the fact that an employer must offer additional consideration (beyond continued employment) in order to bind the employee to a non-compete. It is likely that a Court would require additional consideration for the additional terms as described in your situation. The practical quesiton is what if the employee refuses to sign? Perhaps the employer will then only seek to enforce the orignal document.