The infection of animals by coccidia is the result of an ingestion of water or food contaminated by the infectious stage - the sporulated oocysts . Most of the time it is a matter of hygiene.
Merozoite formation takes place within the schizonts.
Depending on the species hundreds or thousands merozoites are formed. Merozoites of generations differ in size and quantity. We still do not know the trigger for 'male' or 'female' development. Even if you infect a chicken with a single sporozoite - the so called female macrogamet and male microgamets are formed. By the way all sporozoites and merozoites are 'halpoid' - like sperms.