If your really worried I would probably create custom write panels or custom meta boxes tied into custom post types. That way you can make the custom post types based on user role and control what shows up very easily.
For instance you can make a custom post type called “tweens” with the user role of “contributor” so they can write posts but not submit them.
Using custom meta box’s with a custom post type will give you complete control over what kind of data is submitted. For instance if you want to limit the above post type “tweens” to 5 images you only supply 5 image meta boxes, or limit the amount of words per text box, or strip urls or js, or remove the wysiwyg, etc ( anything really).
Some references:
http://codex.wordpress.org/Roles_and_Capabilities#Contributor
http://codex.wordpress.org/Function_Reference/register_post_type
http://codex.wordpress.org/Function_Reference/add_meta_box