I just saw your message in my Visitor Messages and I would like to help you but it is very difficult to give advices without actually seeing those scenes. Also, I think it will be more effective if you put your posting up on the forum so that more people who have experience with the specific breed. With that said, here are my questions and thoughts:
I agree with you that the St. Bernard didn't get much of socialization so that she doesn't know how to co-exist with other dogs. If she is still young and if you are willing to give her socialization, this is what I would do but I would never risk my own dogs' mental and physical damages (too much stress or injuries).
I would let her meet with your 5 dogs one at a time, in an enclosed area (not on leash, since leashed dogs act up more easily) to see which one of them the SB feels most comfortable. It might be an opposite sex one. First few days, she will only be around with that selected dog. In the mean time, you will want to give the SB your house rules (in my house, follow the pecking order, with is based on the seniority in addition to the regular rules and guidance) in a clear and firm manner and I hope your existing dog can be helpful to show her how she behaves in and around the house. As she gets used to living nicely with the selected dog, include your top dog in the group and see how it goes.
Once the top dog accepts her and she behaves as the bottom dog in the pack, you might want to gradually expand the group but again, make sure she takes the bottom's role.
If this doesn't work out, I am sorry but I would not keep her if I were you. I appreciate your effort to save her but your own dogs have the priority. Thank you for what you are doing and good luck on this case.