You said yourself "I just don't want another cat", so why are you considering it? Yes, it would upset Gizzy & Forest to have an unneutered cat in your apt. Even if they couldn't see him, they could smell him. Yes, it has "bad idea written all over it". I know it's hard to see a cat suffer by being homeless, but I strongly recommend that you find a relative, friend that would be willing to foster, or contact some resuce groups outside of Toronto. I wouldn't jeopardize what gains you've made in Gizzy & Forest's relationship by doing this...you've worked too hard to have achieved some level of harmony and tranquility in your home.
"We humans are indeed fortunate if we happen to be chosen to be owned by a cat." -- Anonymous