Don't know if this helps, but here's a pic of my speckled Sussex at a week or three (she's the one on the left) ..feathers look familiar the little "racing stripe" is almost identical to hers. I read you were worried about fighting; you'll get the normal pecking order malarkey going on, but they'll become the firmest of friends if they are raised together. It's lovely how they bond (maybe that's my loved in lot!!!) I won't add anything about brooder boxes etc as I'm in Australia and it's a bit different here lol. We raised 7 chicks in a wire cage with a light; no need for heat anything here lol!
Good luck, and have fun!!
From the research that I've done, the Mareks vaccine does not prevent the disease. What it does do is make any Mareks disease that the vaccinated chick contracts be less likely to cause the paralysis and tumors that the disease usually causes. So, you could have a flock of chicks that have been vaccinated, and still have Mareks disease in your flock. Any incoming unvaccinated birds would then be susceptible. Some hatcheries advise against Mareks vaccination for the small back yard flock. One thing that does help protect the back yard flock is exposure to turkeys, because they carry a less lethal strain. This works similar to the Dairy maids of long ago who were immune to Small Pox, because they had been previously infected with Cow Pox.
