none of my slaves could breed with a minotaur or the salamander since they didn't have multiple pregnancy traits.
In Complex Breeding, that is a stat, rather than a trait.
You can increase the "womb size/Max concurrent conceptions/multiple pregnancy" via the tentacle room with "Enlarge Womb Size" modifier or via womb dilation, which happens randomly during climax inside (Which is how you got bigger womb before room upgrade).
If they don't have the required womb size for monster offspring, they can still have slave offspring if they have the corresponding essence (feral for minotaur, elemental for salamander).
I'm having trouble getting the essences to pop up when I use the growth vat from Noel
Despite having the UI suggest otherwise, I think that you get predetermined traits from Noel. No matter what traits you roll.
On top of that I can't see any changes occurring to my slaves in the tentacle rooms. Enlarge breast size doesn't seem to so anything and draining essence doesn't seem to do anything.
When a negative number is displayed, you need at least that much in "Vitality Storage". You get those via the "drain" modifiers.
When the requirements (Vitality / stat to drain is present), there is a chance that change will occur upon climax. A floating text will appear.