Be very careful if using NiCads in equipment that was not designed for it. A standard dry battery cell has a high internal resistance which means that even if you short directly across the battery terminals the amount of current that flows is relatively small. With NiCads they have a low internal resistance which means that in the event of a short circuit in the equipment there can be very high currents flowing, which can cause things to get hot and possibly catch fire.
I'm not sure about other types of rechargeable cell, but I imagine that they may present a similar hazard.