UA to AC requires a terminal change. If connecting UA-UA at EWR for domestic flights, I try to give myself at least two hours, as even on a normal looking day, things can get backed up quickly.

If you do not have Nexus/GE, I would not plan on making a 1.5 hour international - Canada connection, especially if you know you have to be in line with an agent at customs. Once you clear, you still have to get on the Airtrain to terminal A and then re-clear security. If everything goes right (land on time, with not much of a line at customs, Airtrain working properly and comes quickly, a light security line in Terminal A), it is certainly doable without too much trouble. But one thing goes wrong, and it can easily turn into a missed connection.

Yes, AC will re-book you. But if there is no space, it could take time to get you out. Personally, I would book the longer connection being more sure I could make it, and perhaps, if there is time, see if AC will get you out on the earlier flight.