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Re: why higher impedance current source introduces better PS
With a perfect current source (infinite impedance) connected between Vss and two transistor sources of differential pair transistors, if you look at the small signal half-circuit, you'll see that the potential of sources will be 0 (because of the symmetry), which provides a perfect shield from supply voltage variations. If you have a finite current source impedance Rss, the variation of power supply will be seen (through Rss) from the differential pair transistors. The smaller Rss is, the voltage across it is bigger and so is the disturbance of the bias current and the output voltage.
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