I couldn't agree with you more re: several of your points: I love the productivity record of the Leghorn, and don't even object to their white eggs. You get a large egg from a smaller, feed efficient bird. That's why I've chosen the Rose combed brown Leghorn: Small comb, fuel efficient, smaller frame, well camoflaged. And, these girls can run! I expect them to be about as good as they come at predator evasion. After a season or 2 with these girls, and the Doms, I'll be well positioned to make some decisions about what works best in my flock. I expect that the Doms are here for the long haul. The RCBL may, or may not make the cut. I will bypass any feather footed birds, b/c IMO, they don't do well in snowy frozen conditions, even though common sense might lead you to think otherwise. Buckeyes are on my watch list. I'm thinking that i'll need to choose and stick with either rose or pea combs, and it'd be counter productive to mix the 2. Buckeye x EE might make a fun bird. (Buckeyes are pea combed, aren't they???)
Alaskan, you and I, an I'm sure a lot of other people seem to be heading in a similar direction and choosing the same breeds for the same reasons! Are you putting any broodies into your flock? Or do you have an incubator?
I LOVE Leghorns for so many reasons and because my folks raised Babcocks for nearly 15 years I am very familiar with the White SC varieties. But Mediterranean class fowl are not shrinking violets - yes they are usually mind-their-own-business independent alert foragers but for lightweight birds can really get assertive in their adult years to the point of actually bullying smaller or the larger gentler breeds so I wouldn't put any Mediterraneans in a mixed flock of non-assertive breeds. If your RCBLs turn out great layers than I would hesitate keeping them with gentler or smaller breeds.
Though Dominiques are not shrinking violets either they are less combative than Mediterranean breeds and I wouldn't mix the two breeds. EEs and Ameraucanas are non-combative gentle souls and are really lighterweight than all their fluff leads you to believe so I wouldn't put them in a mix of RCBLs either. Our Buff Leg tore out the beard/muff of our Amer (my avatar) who didn't fight back and they were the exact same age! We had to re-home both our White and Buff Legs - wonderful birds but too assertive around our gentler breeds.
Buckeyes were created with pea combs for dual purpose in cold climates and brood their own young so should do as well as the Dominiques. I just prefer Dominiques because of their overt friendliness and curiosity with human interraction - excellent feed-to-egg ratio, excellent foragers, rose combs, brood their own young, good layers, cold hardy, alert, friendly, no feathered feet or muffs, speckled camouflage feathering that is extremely s-o-f-t to the touch and some use the feathers for fly-fishing, extra cockerels dress out nice, etc etc. I love so many chicken breeds for various reasons but the Dom fits the bill in every category I come up with as either a pet or a utility breed for all climates and productivity.