In-Portal has a set of validation rules you can apply on a field and they are listed here: http://guide.in-portal.org/eng/index.php/K4:Defining_Database%2C_Virtual_and_Calculated_Fields#Validation_Options
Each of the rules works on it's own. However some rules do sort of required checks on field value, e.g. min_len, min_val.
Usually it's up to a developer not to set conflicting validation rules, like 'required' => 0, 'min_len' => 3, in this case. But in this case these validation rules are set internally based on user configuration section.
And yes, it seems that we shouldn't set username length validation if it's not required.