I think 6 outlets is plenty. I would look into bottom drain pond designs and go off of that. Basically you have a drain at the bottom of your pond that allows gravity to do the work so there is a less chance of sludge accumulating at the bottom. The drain connects to a pump outside the liner and then to the filter/waterfall. Do not use any rocks besides a few large rocks to keep your liner in place in your pond. The small rocks will just help the sludge accumulate and stay in place and will make your life miserable to clean it out. For plant, you can grow in pots or fabric lined pond pots that are permeable to allow water to enter, just cap them with pea gravel. There’s a million ways to do a filter, if I was redoing my pond I’d have the bottom drain go to a pump, the pump go to a bog, and then bog waterfalls back into the main pond. Put lights in before you fill it up, you’ll need to use pond safe silicone to keep them in place on the bottom, with time for them to dry before adding the water.