Electrician connected LED dimmer to halogen bulbs, worried about safety
From the link you posted:

The dimmer is rated for up to 210W of halogens with or without transformer. So if you have 6x 20W 12V halogen bulbs, total 120W, it's okay. You should check the power on the bulbs to make sure they're not 50W halogen bulbs, in which case it would exceed the rated power of the dimmer, although 12V 50W are pretty rare.
When your bulbs burn out, you should replace with LEDs. You probably have 12V GU5.3 sockets, there are almost no usable LED bulbs in 12V GU5.3 and they're all super expensive, which means you'll need to get rid of the transformers and replace the GU5.3 sockets with GU10 sockets. I recommend Osram EAN 4058075260115 GU10 bulb, it is cheap, it has excellent color rendition and light quality (much better than halogens) and it is dimmable.