Where have you heard that it can be leaky? Any links?
Marek's can be spread by birds, rodents, the wind...your vaccinated birds can still get sick, they just won't show symptoms but can shed the disease and you'd never know it...or they might. As long as a bird with the disease is alive it sheds virus in dander. So the cute little sparrows and doves that visit every feeding hole around you can bring it if another person has an infected flock. It's not necessarily that you don't have good biosecurity measures, but what about the people around you?I'm not sure what's meant by 'leaky'. I have both vaccinated and unvaccinated birds in my flock, and have both hatchery chicks who are vaccinated, and home bred chicks who aren't, every year. So far I don't have Marek's disease in my flock, but if it arrives, my vaccinated birds should be able to live better, and the unvaccinated individuals are more likely to develop tumors.
Vaccinated birds don't spread disease from the vaccine!!!
Any bird who does have the disease will spread it, vaccinated or not. The goal, and best thing, is to avoid having the disease infect your flock. That's about biosecurity, buying carefully, and good luck.
I agree with what igorsmistress says.I would ask if the chicks are vaccinated, specifically for Marek's. If one batch is and the other isn't, I wouldn't mix them. If they all are or are not then it won't matter.