In regards to the filter, I've always read that you can't "over filter" water, and if the flow is too powerful in terms of water movement you can point it at a tank wall or decor etc to slow it down
This is one of the several myths still being expounded in the hobby. There is no benefit to "over filtration" but there is usually a detriment, to both fish and filtration itself. I touched on the fish aspect previously. As for filtration, a filter can only do what is required by the system; if the filter is adequate for the biological system, it will do the best job of filtering the water...in so far as that goes. Adding more filters, or larger filters, or faster-flow filters, will not improve filtration; it could actually lessen it. Nitrification, which is what we usually expect the filter to deal with, cannot occur if the water flow is too fast through the media. And it will occur at the level required for the biological system provided the filter is adequate, but no more. There is no "more" filtration possible. And besides that, this nitrification will take place in the substrate anyway. And if you have live plants, you lessen the need for the filter even further.