"Peeling" is NOT your problem, if prepped properly there is going to be ZERO problem with anything like that. You simply paint it around the edge hopefully to hard edge somewhere, you would have to post a photo of your firewall to get a good idea. But there is NO problem what so ever with "peeling".
The only thing you really need to worry about has been brought up, that would be color. If you were to buy a pint or quart of paint today and paint your fire wall and then down the road months prep the truck and buy a gallon of paint to shoot it, the firewall will likely be a little different color.
That is really the only problem you will have. If you were to plan to shoot your truck real soon, go ahead and buy all your paint and use a little to shoot the fire wall that would help the situation greatly, though as mentioned if we are talking a metallic color the color still could be off some, but his all depends on your expectations of the finished product. If you are after perfection, it may not make it. If you are after a real nice job but if that firewall were off a tiny bit you are ok with it, you have a good plan.
Post a photo of the firewall where it wraps around the edge, let us see where you could break the color.