Are there any regulations on fueling aircraft inside hangars?
Your previous FBO is correct. According to FAA Advisory Circular 150/5230- 4B Aircraft Fuel Storage, Handling, Training and Dispensing on Airports:

Standards The FAA uses the standards contained in the most recent edition of National Fire Prevention Association (NFPA) 407, Standard for aircraft Fuel Servicing. NFPA 407 provides a standard for the storage and delivery of aviation fuel in an airport environment

NFPA 407, on the other hand requires outdoor fueling.

5.10 Aircraft Fuel Servicing Locations.
5.10.1 Aircraft fuel servicing hall be performed outdoors...
5.10.2* Aircraft being fueled shall be positioned so that aircraft fuel system vents or fuel tank openings are not closer than 7.6 m (25 ft) to any terminal building, hangar, service building, or enclosed passenger concourse other than a loading walkway.

A revision of the same standard also looks the same, with some differences in wording. So, the aircraft fueling should be done outdoors. Its doubly important if the hangar/airport where your aircraft is located also follows NFPA 407.