Following on to what YOWgary says, unless you are planning to shift most of your flying to AC, which does not sound like the case, just keep using your UA number on AC bookings. They will automatically recognize your Star Gold status and give you all the benefits associated with Star Gold - which are not as extensive as the 1K benefits offered on United flights but still worthwhile. If you create and use an Aeroplan account you will have no status and end up with stranded points.

Supposedly AC and UA are forming a closer alliance where we will enjoy enhanced benefits on each other's flights. Very much TBD what that means in practice.
https://www.prnewswire.com/news-rele...wo%20countries.