I'm no expert...
But I believe the flex additive only assists with the initial drying. IE, use it if you paint the bumper off the car and it will flex a LOT in the next day when you install it.
I've been told that flex additive, fisheye eliminator, etc, are all unnecessary... but I'd imagine there's some sort of Great Debate about that amongst the pro's.
I painted my Z in separate pieces in my backyard. I did 1 coat of epoxy and one or two light coats of primer on the bumpers and rubber trim. I didn't use any additives. Had no trouble installing them... then again, not much stuff flexes on my zx.
I also repainted the front bumper of my mazda 626- one of the new style bumpers that is plastic. I washed it, scuffed it with scotchbrite, wiped it with rubbing alcohol to kill the static, sprayed a coat of bulldog (adhesion promoter), 1 coat of epoxy, and then my paint. Again no additives.
Also, I wouldn't use the duplicolor paint. Its lacquer based, I believe. I'd also avoid rustoleum. A budget paint like PPG Omni won't cost much and he'll be able to get the color matched. I've sprayed both the urethane and acrylic enamel... I liked the AU better than the AE, and the AU is only marginally more expensive.