Well, like I said, the pump size is going to be dictated by quite a few things...perhaps the first of which would be the capability of your overflow system to move water out of the tank. For example, I was running a sump on my SW system until recently and my overflow could only move about 600 gallons of water per hour and trust me, the last thing you want is for the return pump to push water faster since that would lead to the main tank overflowing. Conversely though, if the pump in the sump is slower than the overflow system (assuming that system is set up right), then the water movement is limited to the pump speed...or in other words, the overflow can only remove water when the water line is at a certain level. So, as a general rule of thumb, it is probably best to have a slightly over sized overflow system versus having an over sized pump. I know at least a couple people who use ball valves to regulate the return water flow, but I believe your ability to do that might depend on the type of pump you have; at least to some extent.
The other problem you can run into is that the return flow becomes too strong for the inhabitants in the display tank. Personally, I think most fish can handle more water flow than most might think at first, but that doesn't mean I would start dropping a bunch of wave makers and power heads without at least some consideration as to what I am keeping. As always, I can't say that I am even a plant novice, but IME, too much water flow, or misplaced water flow can become a problem as far as wanting to see plants standing up versus bent over or being constantly pushed until they are uprooted. IME with sumps, it is a much more safe practice to keep the sump return flow plumbing near the top of the tank in order to reduce the odds of overflowing the sump via a reverse siphon caused by a power outage (the last time it happened to me, I wound up with about 90 gallons of saltwater on my floor
)
While I don't want to give you, or anyone else, the impression that I don't think sumps or wet/dry filters aren't a good thing (because I definitely do), you might be gaining some extra filtration from the plants themselves...meaning you could save some money. Again, just an idea that you might want to explore.
