disconnect switch amp rating

It depends. You can get a 180 amp switch for around $30. IMO, It will be capable of doing the work you need as long as you don't run starter loads through it..

A 20 amp is OK for just switching other relays on/off. The load in the car from most things downstream is likely going to be greater than 20 amps.

Depending on how you plan to set up the system, I'd look for a switch in the 80+ amp range just to be safe sans the starter loads. There is the minimum standard for a cut off that will make the car legal, then there are improvements. How you plan to isolate the alternator, alternator type, whether you want the big starter cable hot all the time, lots of choices to consider. Pick your method and go.