I'm sure you have already planned your trip but here's my two cents late...
We stayed in a rented room in a family's house (this is a typical accommodation) in the village of Lake Bohinj, which is at the base of Triglav National Park. We didn't go to the top of Triglav as in mountain climbing but we hiked all around the area and there are very well marked trails and huts were you can spend the night. We only did day hikes. Unless you are doing actual mountain climbing and need ropes etc, a guide is not necessary just for hiking.

We didn't book accommodations in advance but that was a long time ago. At that time, there was a tourist office in the village of Lake Bohinj and they had a list of rooms for let. There were also a few hotels nearby but we were on a tight budget. Honestly, the family we stayed with were absolutely lovely! We had a wonderful breakfast in bed each morning and the wife even drove us to some of the trail heads. We didn't have a rental car but that would make it easier for us to get around.