I wouldn't add any leghorn or other strictly egg breed to the mix. The only exception would be Minorca which are a big bird laying very big eggs. Otherwise it would be so tough to get them beefy again and there are plenty of excellent laying heavy DP breeds.
Good choices are Australorp, Delaware, Faverolle, Jersey Giant and Lamona - all big birds and excellent layers. My Jersey Giants rarely miss a day laying.
For heat hardiness you could add in Catalana.
I'm raising black Penedesencas - a DP bird with excellent flavor and lay very well. I'm getting about 90% lay rate and they're fairly heat hardy.
Good choices are Australorp, Delaware, Faverolle, Jersey Giant and Lamona - all big birds and excellent layers. My Jersey Giants rarely miss a day laying.
For heat hardiness you could add in Catalana.
I'm raising black Penedesencas - a DP bird with excellent flavor and lay very well. I'm getting about 90% lay rate and they're fairly heat hardy.