Dear Alessandro:
No matter what kind of batteries you use, you need to match the voltage and current requirements of your camera. You may have a label underneath the camera that shows this information.
First, choose type of batteries that can deliver at least twice the milliamps your camera needs, the more the better, for technical reasons. Then decide how many batteries are required to build up to the voltage the camera needs. If your camera needs 9 Volts, you may want to start with 7 batteries and may be add one more after some experimentation. Better to start on the safe side. If your batter holder is designed to hold only 6, may be that is all you can use, unless you bring the conncection outside.
All the storage batteries are rated at 1.2 Volts. Variations you hear are due to the slighltly higher voltage present right after the battery is charged, and under no-load conditions. But very soon, it will settle down at 1.2 Volts, under load.
When the batteries are suppling rated voltage, milliamps are determined by the device, in this case the camera, not the other way round. It will draw whatever amount it needs. If you are using zoom while running the camera it will draw more milliamps. The more milliamps the battery is capable of supplying, the less the chance it will drop down in voltage. Some devices shut down if the voltage drops drown below a certain level, may be your camera as well.
Coming to your choices, NiMH is the best, as it can supply relatively large amounts of milliamps at constant voltage for relatively long time, and as the batteries can be charged fast. Ni-Cad loses its voltage easily even when you are not using, runs out too soon, and takes much longer to charge. Alkaline rechargeables, in my opinion, are simply a myth. Solar charging, only if you are in a remote corner of the world where there is no electricity. In a bind, I sometimes use UPS as my power source, Uninterruptible Power Supply that is designed for computer use.
Good luck.