I would like to know if VTOL UAVs necessarily need $n$ identical propellers and motors. Have any design solutions considered the possibility of using non identical props? For instance 2 motors of type A, 2 motors of type B, and 2 motors of type C. If so, where can I find some studies or some designs that have attempted this kind of solution?
MIT CSAIL has researched apparently quite thoroughly on this and the end result it would seem is that you can have ANY different size of motors be able to fly given that it satisfies certain conditions such as thrustep etc. For this I would recommend following developments in two areas (as far as my knowledge goes) First of ckurse MIT CSAIL's video demo of their custom software to simulate all combos of motor thrust and positioning
The next 'guy' you want to follow is ofcourse Raffaello D'Andrea and his research team. His TED talk explores drones at mechanical disadvantages, here you go
That's as far as my enthusiast knowledge could help