About combination between QAM and PAM
It is quite common to combine amplitude and phase modulation. QAM is actually a good example because generally you will have symbols with different phases and different amplitudes. Just in the special case of $4$-QAM (which is actually the same as QPSK), all symbols have the same amplitude. But if you take as an example $16$-QAM then you'll see that the information is encoded in the phase as well as in the amplitude:



[Chris Watts [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons]

Note that in general the exact position of the complex symbols is a compromise between ease of implementation and power efficiency. QAM is especially easy to implement and the penalty compared to more power efficient constellations is relatively small.