Hi
Infernalaxe, glad to hear your materials are arriving! First off, I just wanted to remind you that deployment is entirely optional. It's meant as a celebration of the hard work you put into making your costume
While it may be possible to deploy using a plate carrier, as some people successfully deploy using motorcycle armor as an undersuit, it's hard to say if it's deployable until we see it. Using a plate carrier alone and having no ODST-specific armor would not be deployable, as it would be considered an incomplete costume. We like to avoid costumes that look generically sci-fi or generically military given that Halo has a very distinct art style. It may be possible to use a plate carrier and build ODST costume armor on top of it, but I can't say for sure without seeing it in action.
Personally, I'd recommend building the full torso. It's the simplest path to deployment if that's what you're after, and your costume will be better overall for it

. With the torso being the largest part of the armor, I understand that they can feel kind of looming. Sometimes I'll choose to build the torso sooner rather than later so I can get that pressure off. Fortunately, the ODST uses a lot of flat and smooth components, so it should come together faster than a Spartan torso. Plus, I think you'd take more satisfaction in knowing you built it

!
Also note that Deployment requires being a member of your local regiment (Midwest Regiment). For more information about regiments requirements, see here:
New Recruit FAQ