SOA specification during inrush current to the capacitors, or during an accidental short circuit, is equally important, and a very dangerous catch for the young players, like the summer workers writing appnotes at TI.
A lot depends on how you drive the transistors. Is it an integrated power path controller?
What are the worst-case desaturation detection thresholds - at what voltage max, how long it takes to turn the FET gates off?
During capacitor bank charging, you may have a situation of Vds=10V and Id=1000A for some tens of microseconds. Look at the SOA graphs whether this is acceptable.
All the numbers are available or can be approximated: total capacitor ESR, and wiring R actually limits current, RC time constant approximates the length of the inrush.
Then steady state DC. Look at the worst case (lowest) Vgs your FET driver guarantees. Note that this can be abysmally low for poorly designed IC products with too low UVLO voltage - I have seen products that cannot guarantee better than around 4V Vgs, and these products are supposed to be used with high power (non-logic-level) transistors. A huge catch: such products are completely unusable but they won't tell you that, because they want to sell them.
But if you get far enough to know that your FET driver part can supply, say, Vgs=7V in worst case, then look at the Rds(on) curves for that temperature, calculate power loss I^2*R, look at the thermal specification, and calculate the temperature rise dT = thermal resistance junction-to-ambient times I^2R loss. SMD parts tend to site junction-to-ambient thermal resistance at a pessimistic condition using a small SMD pad on a 1-layer PCB with no extra cooling; you are safe using that number, but if you can provide better cooling, you can do much better.
Don't forget to include a traditional fuse. The failure mode for MOSFETs is they blow short.
You may want to add a TVS diode to add extra robustness againt blowing from short-time overvoltages.
As for how to find the maximum required current - if you have no measured data about how much you actually need, how about summing the maximum specified currents of the motor controllers, etc., and adding a bit of margin.