I started out with all Nankins, but since they were so rare (Critically Endangered when we started) I couldn't bear to eat the eggs. It felt like I was eating bald eagle eggs! We hatched out a LOT of chicks those first few years, growing the flock and sharing with other 4H families. I hope we're at least a small part of why Nannies are now off the "Critical" list and are now listed as "Threatened!"
So - the first "outside" additions were bantam EE's. That way I could easily tell which eggs were "safe" to eat. We still have one of the original EE's and several of her half-Nankin younger ladies, all laying beautiful light-aqua eggs.
The next additions were unintended. The elementary school where I worked was trying to rehome chicks from their classroom hatching project. Have you ever tried to say "No" to seventeen earnest six-year-old faces when they pleadingly ask you to "Please, rescue our babeez!" No? I didn't think so, or you would also have a flock of teeny tiny bantams, a New Hampshire Red and a VERY large BJG! I called them my "Kindergarten Dropouts" and they obviously learned the class' "sharing" and "kindness" lessons. They were real sweethearts.
The mixed flock did so well together that, when Herman the House Rooster (his story here:
https://www.backyardchickens.com/articles/is-there-a-rooster-in-the-house.74089/) finally discovered girls, we ended up with "Boo," hatched Halloween, of course! Boo's mama is the BJG. She's a big, charcoal girl with floofy feet and she gets along just fine with the smaller birds.
So ... the current flock consists of about a dozen Nankins, three mid-sized EE hens, Boo(the gentle Giant cross) and a lone Sapphire Gem rescue. The only issues I usually have revolve around chick integration - and those involve my sassy little top hens, not the "Big Girls!"
I will note that you'll need to make sure that any roosters you keep with a mixed flock are bantams. Standard males can seriously injure smaller hens when mounting. When Herman was out with his ladies, I kept them separate so he wouldn't "lovingly" hurt my really small hens. Both of my current roosters are full Nankin.