From a technical point of view it doesn't ring true, people are just not heavy enough to act as ballast on a sailing ship, even if you laid them one on top of another right down in the bottom of the ship.
How many tons of people would you need to effectively balance each ton of coal? Far easier to put the coal in the hold and charge the people to ride on top. In any case you don't need to feed 20 tons of rock.
It's worth remembering that the causes of cholera and similar diseases were not fully understood at the time, Cholera was endemic throughout Europe and outbreaks were common on all ships regardless of the passenger manifest. However the cheapest emigrant ships were also the slowest, consequently although emigrants already infected certainly took passage an all ships the longer crossing time gave the disease a greater opportunity to take hold.
All the best