How to determine the highest dominant pole?
This circuit is a MOS Cascode Amplifier
First of all, you should find which capacitors contribute to low frequencies.
To find this you should test individually for every capacitor if we replace it with open circuit, the gain reduced. If this is true then you understand that, this capacitor contribute to low frequencies.

The methodology that you refer above is correct.

fp1 = 1/{2*π*Cc1*Req1} = 588mHz
Cc1 = 1uF
Req1 = 4k+ 800k//400k = 270.67K

fp2 = 1/{2*πCSReq2} = 374.53Hz
Cs = 1uF
Req2 = (2k)// (1/gm1) = 2k // 540 Ω =  425.2 Ω
where gm1 = kn'(W/L) (VGS-VT) is the resistance when you looking the source of the M1(assuming λ=0 i.e ro infinity).
To find gm1 maybe you should analyze the circuit in DC and find that VG=4V,VS.Afrer calculation gm = 1.85mS and 1/gm = 540Ω, ID = 0.86mA   

fp3 = 1/{2*πCGReq3} = 48.6mHz
CG = 10uF
Req3 = 1800k//{(4k//800k)+400k} = 329.93k   

fp4 = 1/{2*π*CC2*Req4} = 15.92Hz
CC2 = 1uF
Req4 = 4k+6k= 10k
if λ&lt;>0 then Req4 = 4k+ {6k//(ro2+ro1+(gm2+gmb2)*ro2*ro1) }= ...

As you can see when Req increases pole decreases.


  So the fL = fp2 = 374.53Hz


This online pdf I think will help you http://aries.ucsd.edu/NAJMABADI/CLASS/ECE102/12-F/NOTES/ECE102_F12-LecSet-8.pdf