How should I deal with a chicken at the bottom of the pecking order?

[NatJ]

Peepo is 3 1/2 years old... 18 weeks... There would be a huge age difference and I have read that it isn’t a good idea to put chickens of different ages together.

You can figure that all adult chickens are the same "age" as each other, for this purpose.

Given how you've described the current situation, I would be inclined to just keep doing what you've been doing--it obviously works fairly well, and Peepo isn't really all by herself. New chickens could make it better or they could make it worse, and it's really hard to predict which way it would go.