Boy oh boy is this going on and off course. The OP bought the correct LP detector and it is supposed to be installed at least 6 inches above the floor for a surface mount. Prevents damage by vacuums etc.
The combo smoke/CO detectors should be mounted high in a room but not on the ceiling. I don't believe that 120vac combo units are plug in. They require a 4' round box with approved 120vac wiring. Check NEC.
Battery powered alarms are allowed, at least in California and depending on jurisdiction may have to be hard wired. I just went through this while selling a second home last month. There is a mix of hard wired smoke detectors and both CO detectors are battery powered.