As an ultralighter,I don't think you need any extra layers for your legs for those temperatures. I've done plenty of hikes with mornings in the 20s and 30s in just thin nylon hiking pants. Basically, if you're cold, you should either be walking to generate heat or in your sleeping bag. That way, you wouldn't need any extra layers during the day--just whatever you sleep in (and a lot of long-distance hikers just sleep in their hiking clothes, though I prefer thin tights just to keep my quilt clean). If you're expecting a bunch of rain, you could bring rain pants, and you can wear those as an extra layer if you're really cold. But I wouldn't bother unless the forecast was calling for a lot of rain. Rain pants aren't very comfortable to hike in, and if they are waterproof enough to keep the rain out, they will keep sweat in, even the breathable ones.
Also, zip-off hiking pants will be more versatile and lighter than shorts + tights.
That said, clothing is very individual, and it's hard to know how warm/cold someone else will feel in the same conditions and clothes that I wear. So you may just have to do some trial and error here to figure this out.