CBsubs - Different fields for different plans upon registration

Upon user registration, is it possible to show different fields based on the chosen membership plans?

CBSubs can not show fields based off selection, but instead hides them. So you could basically use 2 plans to toggle the visibility of fields. Example as follows.

Plan 1 - Exclusive
- Hide Fields A, B, C
Plan 2 - Exclusive
- Hide Fields D, E, F

Fields ABC would show if you selected Plan 2, but hide if you selected Plan 1. Fields DEF would hide if you selected Plan 2, but show if selected Plan 1

And can I hide on plan A fields that are mandatory for plan B?

Yes.

This thing that you can't associate fields to a plan is quite counterintuitive, and doesn't work if as in my case you have a mandatory plan

Free - the default registration, needed to have a fallback plan when a paid plan expires. Here I can hide some fields
Paid Memberships: here I should display a disclaimer field, but it doesnìt show up since it is hidden on the mandatory one

How can I configure the whole thing to make it work?

Have the free and paid plan be exclusive from one another. Have the free plan hide the fields of the paid plan. Next use CB Auto Actions to subscribe them to the free plan when the paid plan expires using the CB Paid Subscriptions action with the below usage.

Plan Active
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Equal To A

Plan Expired
Global
Triggers: onCPayUserStateChange
User: Automatic
Access: Everybody
Conditions
1: [var3] Equal To PLAN_ID_HERE
2: [var2] Not Equal To A

Believe that should give you the fallback behavior you're looking for while being able to hide fields based off plan selection.