I suspect if they're all drifted up about the same, you'll be OK because the voltage divider ratios will be about the same.
Not seeing much in the 1+ watt megaohm value range at Mouser
The reason they used 1 watt is because they have a higher breakdown voltage than 1/2 watt resistors. They strung a bunch in series for the same reason.
Typical limit for 1/2 watt is 350, 1 watt 500 and 2 watt 750 volts.