Halogen transformer replacement: safe to use 60w max to replace a 75w max

AFAIK, bulbs with a GU-10 base are driven directly from electrical mains, in which case it's OK to use a lower-power bulb. Your fixture won't reach the temperatures that it's capable of withstanding.

The only place that I know of that it might not be OK is with low-voltage (usually 12V or 24V) lighting systems such as track or task lighting: some transformers have a minimum power output that they like to supply, so if you use lower wattage bulbs, you should use more of them to compensate (somewhat defeating the purpose). If this applies, you would have an entry for Minimum power on the specifications for the lighting system.

Unfortunately, my lack of Dutch comprehension prevents me from giving a definitive statement on your situation (do you have an English translation for those switches?). That said, I live in England and all of the halogens (with their transformers) in my flat are on dimmer switches, so it is absolutely possible to have dimmer switches on your lights.

However, you are going to have to make sure that the transformers that are currently installed in your house will work with a dimmer switch (it is possible that yours will not) by checking the manual or looking it up on the internet.

And definitely take care when working with your electrical system. If you at all feel uncertain or uncomfortable, it may be worth the peace of mind to talk to or hire a professional.