I just got off the phone with a city of Minneapolis employee who was intimately involved in the ADU ordinance language and permitting for ADUs. Sewer can definitely be tied into the main house for a detached ADU. The only requirement is that you have a backflow preventer in the ADU. Bloomington requires a separate connection to the sewer system, and they had many talks with them. The city found that a separate connection, which would have also triggered the SAC fee, would make a detached ADU very cost-prohibitive.Sewer most be hooked up on the mainline. Believe me. Water and gas can come off the house.
She also warned about lower flow if you tie into the existing water pipes. Unless you've recently built a home, the pipe from the main to the meter will potentially be under-sized for both a home and ADU (depending on occupancy, usage, etc). So that may be a consideration, either up-front or down the line.
I also asked about gas and she said they didn't contact Xcel or Centerpoint directly but didn't expect there to be an issue.