I've seen people cut off lower branches on conifers so they could put stuff under it. Is this a bad idea for a 30 foot tall tree?
Not a bad idea, unless you wanted a tree with branches close to the ground. You can do pretty much whatever you want with trees as long as you don't do anything that will cut off oxygen from getting to the roots (e.g., pave over them, raise the ground level) AND always leave so green above ground. WYSIWYwG.