You don't have to place the monsters at the very end of the first day. In ideal, controlled conditions, you can place them on the second day... Well, considering the resource search and protection against the absence of wolves, you can assume it's around day 4-5.
I'm talking based on the assumption of "160%" difficulty. It seems they don't play a significant role on larger maps.
It's just that monsters are really cheap in every sense, and they have few requirements. It's not like you need to upgrade giants.