Friggin basic stuff!
A 100amp TP isolator in a DB is rated at 100amp per pole. That's it, the end.
If it was a four pole version, each pole again would be rated at 100amp.
So, in your judgement, if you drew 100amp on each phase, how much would your 100amp neutral pole be expected to carry?
When you buy a 32amp 3 pole rotary isolator from the wholesaler, do you believe it is only rated to 10amp per phase?
If you fitted a 63amp TP MCB (80amp MCB's are available, but not for the majority of off the shelf TPN boards), then what rating do you think it will trip at? 63amp overcurrent on one phase, all phases? or 20 ish amps on one phase?
I would walk away from this job if I were you.