You can use a stronger pump for your waterfall, it is all on how you stack your rocks and how you lay out the liner for the waterfall. If you find an area that is leaking you can put goop under a rock, stick it where the water is over flowing to redirect it. Or use the expanding foam to direct the flow. My liner goes out around a foot past the water fall area on each side and curves up to collect any water that goes the wrong way.
My flagstone does not feel grainy so not sure.
I have a skimmer, it is installed, but not the final attachment to the pump yet, it will be plumbed this year before the leaves begin to fall. We have three mature trees around the pond. I will have my pump plumped to either draw completely from the skimmer, from the bottom or a combination depending on need. Esp when those trees start to drop their leaves. Last year most of the leaves missed the pond, must be the air flow pattern over the pond, hopefully the same this year. Just used a hand net to pull the leaves last year, the few that hit the pond.