Need help modelling the circuit with parasitic capacitance of a power supply in this scenario in SPICE
The two capacitors near the InAmp remove common-mode AC content caused by the 1 nF in the SMPS modelled to the left. Some SMPSs will have significantly less than 1 nF and may be tamed by 10 nF decouplers but other SMPSs might have 2.2 nF internally and will need greater help hence 100 nF or greater may be needed.

Bear also in mind that if 1 uF capacitors are needed thay will reduce the output bandwidth of your source because of the 100 ohm balanced drive.

Typically, 1 uF and 100 ohm produces a cut-off frequency of 1600 Hz. However, if you are only worried about slow speed signals from your source 10 uF capacitors can be employed.

One thing to watch though - those 10 uF capacitors will produce a differential signal if they are not perfectly matched.