Is it possible to change the default class "string" to both input and textarea to be my own default class, which I can choose?
And by that I mean simple_form.rb in my initializers.
Yes, you can.Refer to the user inputs section of the simplest forms of gem README.
According to the latest build for the gem, you can change it in your initializer by typing config.input_class = 'your-class', but this will affect all classes of form input fields.
config.input_class = 'your-class'