Tell me anyone would not automatically think of
Nicole upon hearing
Nikki? I'm all for creative NNs, but to just take a well established NN from a different full name is just clumsy and desperate.
I don't think
Monica necessarily needs a NN, but if I were to use the
Nikki sound, I would spell it Nici so that it at least seems logical. Nica or
Mona would work as well.