Having had a Heat pump system in a DIY hybrid configuration for about 5 years now .. No experimenting it just works. I got the house rebuilt in about 2007 and while I had requested a pellet burner, my builder convinced me otherwise. ..and put in an oil burner . So mixed radiators and underfloor and oil burner with mixing valve etc all in place. Eventually I got a Heat pump installed in parallel . There was some complex mixing blending valve put in by the installer to allow the oil burner come on under certain circumstances. Unfortunately the combination was not greatly successful. I then basically removed the blending valve and connected the oil burner in series with the heat pump with the flow going from the heat pump into the oil burner and then into house system and the return water going into the heat pump. That way any very hot water from the oil burner went directly into the house, and protected the heat pump. I added more insulation around the boiler .. except at its air intake.
Bottom line .It works a treat. My stats are that the heat pump has run for 17,000 hours .That is the compressor heating water ,and the oil burner for 92 hrs . So as you might imagine, the oil fills are very rare. The oil burner and boiler adds mass and thermal capacity to the heat pump system, so it has less frequent starts and stops .
The major trick with a heat pump is to try and reduce the average temperature that it needs to heat the water to, and this means reducing as far as possible all thermal barriers in the spaces you want heated. .. So all radiators full on, all floor heating full flow. .. Counter intuitive yes .