Coat of plates / Surcoat of plates doesn't need nearly as much shaping as a single-piece breastplate does since it has more flexibility to it. It's not flat, but it's not the complex deep curves of solid cuirasses.
You can work galvanized cold, you only really need heat for extreme dishing like raising a one-piece sallet or serious stretch-work. Heat certainly makes things like dishing knees or elbows easier, but it's not necessary.
The "brittle" of cold working is called "work hardening".. basically, by hitting the metal enough, it will start to harden up, and gets slightly harder to work as time goes on. Heating will bring the temper back to where it's easy to work, even after it's cooled off. Stainless steel is a *huge* pain with work hardening; the only stainless breastplate I ever made, which wasn't extremely dished, I think had 3 or 4 cycles of heat before it went completely hardened. If you listen, you can tell a difference in the hammer ring if it's hardened completely.
Unless you've got the tools to work the metal with, it's irrelevant. Working something small like coat of plates pieces should be reasonable enough since they're pre-cut and pre-punched, and just need a little shaping.
I'm far from good with armor, and never made anything I'd consider wearing. I can work straps and replace rivets, but that's about it ... I'd ask the guy selling the kit what you should have to assemble it. It might be as simple as a piece of metal pipe and a cheap bodywork hammer for planishing.
