How to clean a power rail for sensitive ADCs
Also for inputs above 5v there is nothing to worry since the PSRR would kick in

This statement is untrue. LDOs take time to react to transients/noise at their input. Frequency/bandwidth/transition times is what is important here, not amplitude.
The PSRR of linear regulators and especially SMPS are not going to be effective against switching harmonics (such as those an SMPS).
Most definitely use a good ol' series resistor (probably 1 or 2 Ohms) before the decoupling cap for the regulator and the voltage reference where voltage is going to be dropped anyways. But you may also want them for each ADC as well. You could use ferrites for the ADC if voltage drop is a concern but selection is pickier than resistors and could make noise worse due to noise peaking.