I made that mistake, once. I thought my Columbia Ice Dragon (Titanium/OmniTech) jacket with the fleece zipped in would be warm enough. After all, it weighed almost four pounds! Not a chance.
The rain/wind jacket should handle rain and wind well when you're in camp. I can almost guarantee that if you hike in it that you'll be sweating like crazy and end up soaked, even without the fleece and with the pit zips open, regardless of temperature.
There are two separate issues: waterproofing (for moisture control) and insulation (for temperature control). Waterproofness is still being "cussed and discussed" here on BPL heavily, with solutions ranging from hoping that a water-resistant (not waterproof) breathable windshirt will keep you somewhat dry, to waterproof/breathable miracle fabrics (eVent, EPIC, not GoreTex) that might keep the inside of your jacket a bit less wet than the outside, to waterproof coverings that try to keep you dry underneath by heavy venting (ponchos, capes). I don't think there is a good answer yet. Since I hike hot when wearing a jacket, regardless of air temp, my solution is the Six Moon Designs Gatewood Cape (it also makes a nice solo shelter). But there are a bunch of excellent alternatives that are very lightweight and affordable; check Campmor's rainwear section. If you don't sweat very much, then stick with your jacket. It will work as well as any other jacket, most likely.
The other issue is insulation. You're right: the fleece is heavy and bulky, and not very warm either. When it's below freezing you're going to wish you'd brought a "puffy": something thick, warm and dry. There are many good down or synthetic 'sweaters' available. My preference is a Patagonia Micropuff pull-over; synthetic, lightweight, water-resistant, and warm. Down works well too, as long as you can keep it dry.
The title of this thread, "Layers," is the key. Start with a wicking breathable base layer, and maybe another slightly heavier layer. E.g., I wear a lightweight polypro tank or tee for hiking, and add a Patagonia R.5 or Icebreaker wool mid-layer when it's chilly on the trail or in camp. These are breathable enough to keep you dry when you're working. Your long-sleeve polypro shirt might be a good compromise for both layers, but it might be worth adding a mid-weight layer anyway; it gets cold in the Tetons. An ultralight windshirt (3-4 oz) is my solution for that biting wind on the trail, or keeping my warmth during a rest stop. If you just have the long-sleeve shirt, a windshirt would be even more useful to extend your comfort range (unless it's raining, in which case you'd be wearing your jacket or something.) My Patagonia Micropuff comes out in camp, and I wear it until I'm ready to start on the trail in the morning.
Anyway, I hope this gives you a few ideas for clothing that will keep you warm, mostly dry and not weight you down too much.