I wouldn't vent the veg chamber into the flower chamber. I would find some other way of keeping things warm. I tried this once and it was a disaster. With HID lighting heat is a constant issue, and I would not intentionally add heat to any chamber. There are other ways of keeping things warm at night, like a heat pad, thermal blanket, or plant heat mat. You can also have some true green lights at night to keep things warm. If you buy the right ones, the ones that are made for this purpose, the light is nonexistent to plants, and a couple would warm up that little space nicely.

Just my 2 cents worth. If you decide to go ahead anyway, just make sure you've got some big-ass fan for the top section. Glad you chose a cool tube.

Looking forward to actual groundbreaking day.