The quercitin would replace the benadryl, being a strong anti-histamine, but you would want to give the quercitin, and then slowly decrease the benadryl if that's what you want to do. Watch the tumor carefully during any changes in protocol. Definitely start anything one at a time and observe response. I don't know of any reason why these can't be given together.
I'm glad the thread has been helpful.
I should say, the tagamet is an immune stimulator. If there is any type of autoimmune diease, there might be an aggravation. My GSD couldn't take it because it made his paw well, but he wa a medical mess. This is precisely though the property which makes it a powerful cancer fighter. Very few dogs have problems with it.