Integration pen is great. I have a little 'bitty one' door in my coop walls that my bantam silkies scoot through no problem. I've got two Tom turkeys, a turkey hen and her two chicks and her teenage son and two bronze turks, a dozen Rhode Island reds, two leghorn ladies, 4 barred rock crosses, two amaracanas, and four barnyard mix chickens. They all live together, sleep together and are different sizes and ages. I have some little trees strategically placed for little bitties to tuck into if they have need. I think space is key. If they feel like they have to compete I think they will but so far so good.
One last update. Everything worked out very well. The little one is wary of the big ones, but they get along well enough, and she's clearly much happier with company.