Different NiMH batteries have various charging/discharging and internal resistances. This makes some of them VERY unacceptable for RC use. I would also never use a battery holder in anything other than an electric plane due to the trouble the vibration could cause in a nitro/gas plane.
Soldering on regular cells is also a bad idea as it can cause thermal damage to the cells very easily.
More battery info:
http://www.rcbatteryclinic.com/