Most likely, the reason the PB put in the large pump was for the water falls since they need quite a bit of flow rates. So you may still need that size but a 2 speed would help a bit on energy costs.
I don't have a two speed but many on this forum do and have not complained about reliability so I don't think that should be an issue. Technically, there is no reason why a 2 speed should be less reliable than a 1 speed. A 2 speed has additional set of windings for low speed and I would think that low speed is not as hard on the bearings as high speed so in some cases it may actually last longer.
Solar is usually not an issue with 2 speed pumps as long as you can prime the panels at high speed. Most automatic controllers have this priming feature with solar so they should take care of it but you might want to double check to see if the feature exists.
If you don't have a controller that will prime the panels at high speed, then you must make sure the pump at low speed has enough head to prime the panels. For a one story house, the pump may need at least 20-25 feet of head to push the water to the top of the panels and fully close the vacuum release valve in the panel. If you know the height to the very top of the roof, add 5-10 feet to that and that should be enough total head for everything. However, most 2 speed pumps at low speed will come up short of this requirement.
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