sorry for bumping this old thread, but i want to share how i solved my problem.

sadly i don't get it done without a plugin. to be able to add multiple subclasses to one gridview, i use silverstripe-gridfieldextensions this collection of gridfield extensions also includes one to sort the entries. ([Nobrainer Web] uses the same plugin in his solution)

<?php

class BaseModule extends DataObject{
    static $db = array(
        'Sort' => 'Int',
    );
    static $has_one = array('Page'=>'Page');
    static $default_sort = '"Sort"';

    function getCMSfields(){
        $fields = parent::getCMSfields();
        $fields->removeByName("Sort");
        $fields->removeByName("PageID"); // get automatic value for current page
        return $fields;
    }

    function getContent(){
        return $this->renderWith($this->getClassName());
    }

    function getType(){
        return $this->getClassName();
    }

    function getShort(){ // not really necessary, but nice to have
        return "";
    }
}

class ImagesModule extends BaseModule{ // example module 1
    static $db = array(
        'Headline' => 'VarChar',
    );
    static $has_many = array(
        "Images" => "Image"
    );

    function getCMSfields(){
        $fields = parent::getCMSfields();
        $images = new UploadField(
            "Images",
            "Images",
            $this->Images()
        );
        $fields->addFieldToTab("Root.Main", $images);
        return $fields;
    }

    function getShort(){
        return $this->getField('Headline');
    }
}

class TextModule extends BaseModule{ // example module 2
    static $db = array(
        'Headline' => 'VarChar',
        'Text' => 'HTMLText',
    );

    function getShort(){
        return $this->getField('Headline');
    }
}

class Page extends SiteTree{
    static $has_many = array(
        "Modules" => 'BaseModule',
    );

    function getCMSfields(){

        $gf_config = GridFieldConfig_RecordEditor::create();
        $gf_config->removeComponentsByType('GridFieldAddNewButton');
        $gf_config->getComponentByType('GridFieldDataColumns')
            ->setDisplayFields(array('Type' => 'Type', 'Short' => 'Content'));
        $gf_config->addComponents(
            new GridFieldAddNewMultiClass(),
            new GridFieldOrderableRows()
        );

        $moduleField = new GridField(
            'Modules',
            'Modules',
            $this->Modules(),
            $gf_config
        );

        $fields = parent::getCMSfields();
        $fields->removeByName("Content");
        $fields->addFieldToTab("Root.Main", $moduleField, "Metadata");
        return $fields;
    }

}


class Page_Controller extends ContentController {
}

to see the output add this statement to Page.ss

<% loop Modules %>
    $Content
<% end_loop %>

and create the templates for the modules (e.g. TextModule.ss) in the Layout-folder.

i'm still new to silverstripe, so perhaps there is a shorter/better/more simple solution.