I think there shouldn't be any conflict between Octavia's romance route and Cathy forming a harem. Octavia's dominance route, on the other hand, shouldn't conflict with Cathy's harem, because Octavia in the dominance route remains a queen with a strong desire for control. However, the romance route Octavia is different. The protagonist enters her heart, and she shows a different side. It's also clear that the protagonist is healing Octavia's inner wounds. Therefore, I think there shouldn't be any problem with Octavia's romance route and Cathy forming a harem. :-)