The quest log entry for the quest should be "Bring Melissa a Taurus girl with huge lactating tits and at least three additional pairs of tits. Size and lactation can be altered with certain potions, while the laboratory lets you add and develop extra tits. ", which explains all the parts needed, but note that the wording here is very specific. It is likely that you had the right number of extra nipples, but the number of extra nipples is not necessarily the same as the number of extra tits. You will need to use the "Develop nipples" operation in the laboratory to change the extra nipples to extra tits.
I'm not that familiar with the mechanics of this mod, but I suspect that there is no method of having slaves impregnate each other outside of the sex interaction with the player character.