I've only used "ski" type pants before that had a little built in insulation, and have been fine in pretty cold conditions with just a base layer under them. I have since gotten some "snowboard" pants that don't really have any insulation, just some mesh on inside. Seems like it might not be enough with just a base layer, maybe it will be.
Before my trip out west I'd like to get everything I need. No real way to test this here in FL as it's been 80*... What if anything do you need to wear as a midlayer for uninsulated pants?
A mid layer on your legs isn't usually necessary, even if you're from Florida (I live in Georgia). I have worn 'warm up pants' before, but now usually just wear a base layer and shell.
Whatever you use should be:
1. Not very thick
2. Not water proof.
You might be better served by wearing boxers under your base layer (or impact shorts over them), maybe throwing on a pair of soft volleyball pads for your knees (make sure they aren't too tight) and calling it a day. Between shorts, knee pads and boots, there won't be much of your leg that isn't covered by 2 layers.