We know ED-E stays canonically, but does anything really rule Cass out? I know her survival is dependent on quests taken, but here's the thing:
All but two of her endings leave her fate ambiguous. She either dies, gets convinced to leave, or her ultimate fate is not revealed. It's plausible that she stays. In fact, in the NCR ending, she seems to have intended to sleep with a male Courier in celebration, although she ends up having six with an NCR trooper. If she liked the courier enough to change her mind about attraction towards a male version, I don't see any reason she couldn't stay. In fact, her ending slides imply she was even with the Courier during the Second Battle of Hoover Dam, or at least nearby and watching.