On the closeness, to me that doesn't seem to be a problem. I can't even recall how many pictures or videos that I have seen showing troops bunched up like that. Though I am not sure the guy immediately behind the left rear bumper with the vest open, that probably won't fly. It would help if they had IBA vests (Dragon's US Marines is excellent and I highly recommend it as others have here), but since PAGST was still out there, especially early on, lets just say that they are National Guard troops
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For BDU colors I use is Tamiya buff, Tamiya NATO green, Tamiya NATO Brown , and ModelMaster flat black. I recommend getting a thinner tip brush and buying a real BDU blouse as a reference and just mimic it. The cloth pattern is the same on all BDUs, but its position just may be different (ie one distinct squigly is always next to another distinct squigly, it just may be on the arm of one jacket and on the back of another., if that makes sense). Right now, especially the black parts look too thick. Also for the DCU, I generally add a little white or sand color to the dark brown to fade is out a bit, just a future tip.
Your concept looks good and has a lot going for it. I would definitely like to see the finished product. Keep it going.
Jeff