Most PICs only have ONE ADC which is multiplexed to a whole heap of pins. This means you have to select your Input, perform an ADC operation, select the next input etc etc...

So, with 6 inputs, your 15uS slot becomes 3uS (15uS/6). With all the other processing, variable comparison, PWM updates etc, it's just not going to happen (at least not with a 12F, 16F or 18F device with PBP - even at 40MHz).

A splattering of small surface mount Diodes, Capacitors and Resistors suddenly become very attractive.