California grey is another breed to consider. They do not have a huge comb, they are barred like a barred rock and lay a white egg. They have a very good feed conversion,too.
If they have both breeds on the same day, it is easy to tell them apart. The leghorns will be dainty little peepies compared to the chunky cornish x . If you are still worried get a brown leghorn chick.
And leghorns come in all different colors now. So you can have a flock of all different colors AND lots and lots of eggs. There is at least one thread on this site about people who breed 'colored' leghorns. They come in black, red, white, exchequer, mottled, barred, light & dutch brown, buff...