Heroes' card texts states that you are limited to 1 hero per zone.

Would certain tactics such as Forced March or Call of the Brayherd override this text?

If I use Call of the Brayherd and let's say of the 5 revealed cards, 3 are units with a cost of 3 or less, and two of them are heroes. Do I place both of them into my battlefield? If not, what's the protocol?

Those tactics don't override it. See the FAQ on Rip Dere Eads Off