I think i've found a solution.
1. Enable 'membership plans' and 'listing packages' under Account tab in admin panel.
2. Create listing packages and assign them to account type A.
3. Create a FREE membership plan (UNLIMITED) with the option to 'add listing' disabled, and assign it to account type A.
* Now account type A will have a membership plan by default, but as it can't use it to add listing, the system will show it the assigned listing packages.
4. Create membership plans and assign them to account type B.
* This account type will get to choose from the membership plans only.