The use of a battery charger designed for Nickel-Cadmium cells and then used with Nickel-Metal-Hydride cells is not a problem. Most of those battery chargers will work fine with both of those two battery chemistry types.
A few words about Lithium-Ion batteries: The Ni-Cad/Ni-MH battery chargers are not compatible with the newer Lithium-Ion battery cells. You must get and use a charger specifically designed for that battery chemistry . You might be surprised at the price for one, but keep in mind that the Lithium-Ion batteries do need to have special precautions observed if they are to work safely. Yes, there have been fires not only with laptop computers but even with cellular telephones with that type of battery. Since the recognition by the battery industry of the need for special protective circuitry with these batteries, this kind of thing is now back down to the "rare" incidence level again. Just be careful of what you are buying with the Lithium-Ion type batteries. If the price really seems to be a bargain in comparison with what you have been paying, it is possible that this particular manufacturer may be cutting costs by leaving out of that battery things that really should be in there.