If I sign the boilerplate severance agreement, am I still protected from retaliation?

I've had both good and bad bosses before, but I really suspect this last one of being seriously unbalanced. Given a range of behaviors including but not limited to his totally denigrating his entire leadership staff to me in our early meetings before I had even met them, things like "X stinks. Y is incompetent. Z is lazy. etc." - absolutely none of which were true, plus his treatment of me, where he thought I walked on water the first 4 months and then did everything in his power the last 2 months to get me to quit, I fear he'll continue to badmouth me as I search for future employment. In other words, he's a pathological narcissist who sees people as all good or all bad and personally has to win every contest. He ultimately fired me, without cause, and I called him on several illegal actions, i.e. manipulating me into OT for which he wouldn't pay me, promising to "make it up" to me. I got some satisfaction on the issue of OT, but I know I infuriated him. My concern is to collect the $4500 (of the $7500 owed), I must sign a legal document saying I would not ever sue them. My former boss, like many narcissists, is quite charismatic and quite active in my industry and region, and personality defects aside, is quite intelligent and hard working. How do I protect myself during my job hunt? Do I ask for a clause to be added to the severance agreement? I only have a few days to make this decision.