Finally I have find the answer (I left here for documentation),
You can create as many themes in Symfony config.yml as you like, so first is duplicate the desired theme and add the plugins you need:
advanced:
convert_urls: false
file_browser_callback: 'elFinderBrowser'
plugins:
- "advlist autolink lists link image charmap print preview hr anchor pagebreak"
- "searchreplace wordcount visualblocks visualchars code fullscreen"
- "insertdatetime media nonbreaking save table contextmenu directionality"
- "emoticons template paste textcolor"
toolbar1: "insertfile undo redo | styleselect | bold italic | alignleft aligncenter alignright alignjustify | bullist numlist outdent indent | link image | print preview media | forecolor backcolor | stfalcon | example"
image_advtab: true
templates: "/admin/newsletter/templates/templatelist"
entity_encoding : "raw"
fullpage:
convert_urls: false
file_browser_callback: 'elFinderBrowser'
plugins:
- "advlist autolink lists link image charmap print preview hr anchor pagebreak"
- "searchreplace wordcount visualblocks visualchars code fullscreen fullpage"
- "insertdatetime media nonbreaking save table contextmenu directionality"
- "emoticons template paste textcolor"
toolbar1: "insertfile undo redo | styleselect | bold italic | alignleft aligncenter alignright alignjustify | bullist numlist outdent indent | link image | print preview media | forecolor backcolor | stfalcon | example"
image_advtab: true
templates: "/admin/newsletter/templates/templatelist"
entity_encoding : "raw"`
After that, in the controller, change the theme you are rendering tinymce with:
->add('content', 'textarea', [
'label' => 'Contenido',
'required' => false,
'attr' => ['class' => 'tinymce', 'data-theme' => 'fullpage']