Just took a quick peak at some MOSFETs through mouser's site (10-20A MOSFETs). Many of the MOSFETs seem to have a gate to source beakdown voltage of 10-20v. Assuming the source is connected to ground, as long as your gate actvation voltage does not exceed the breakdown voltage, the MOSFET should be fine, assmung I remember my class correctly...
As long as your batteries total to lower then the gate-source beakdown voltage, you should be fine. Make sure to use the peak voltage of the fully charged battery, so 4.3 v fo Li-ions and 1.5v for NiMh for a little bit of buffer.
They aren't too expensive at $0.40-$1 for one, not cheap either, but you could test with a few to see how well you like them. Check the specs before you buy of course.