The positive RDS tempco helps at steady state, but keep in mind that often in order for the tempco to be positive you have to drive the gate with some minimum voltage. Check that voltage on your datasheet.
You'll have to watch out during the switching transient though, often the effect of temperature on switching times is not so simple. Preferably you'll want them to all switch equally. Having the cooler devices switch faster should be okay as well too. Worst case is hot devices switch first.