Hi,
I don't see why a 55 watt 12 volt panel would not be OK.
I agree that maybe they really mean minimum 15 watts, which would make sense.
You can run the pump direct off a PV panel and basically use the PV panel instead of a differential controller to run the pump at the right times.
This does not work as well as using a differential controller because 1) the PV panel and solar thermal panel have efficiencies that go in opposite directions with the temperature changes -- so, if its set up right for medium temperature day, then on a cold morning, the PV panel will start the pump early because the PV panel is more efficient in the cold the the thermal panel is less efficient in the cold, and 2) the differential controller knows how hot the water in the tank is and only turns the pump on when the collector water is above the tank temp. The PV panel does not know the tank temp, so it may turn the pump on to early if the tank is already warm, or too late if the tank is cold.
All that said, the PV panel direct to pump works pretty well.
ArtTec sells a differential controller that is made for use with pumps driven by a PV panel.
http://arttecsolar.com/Can you give us a link for where you purchased the pump? Looks like it might be a good choice.
Gary