The current fieldset implementation in it's standard form is quite limited when it comes to custom html, you can only play a bit with the template values in the form.php config file.
If you need more complex layouts, you'll have to "break up" your fieldset, and start generating fields seperately.
and so on. You can also enable and disable fields in the fieldset, so you can generate more complex forms.
This will be adressed in Fuel v2, which will have more object oriented approach to form building, with custom renderers to give you a lot more flexibility to generate more complex forms. You can have a peek at it here: https://github.com/fuelphp/fieldset, some already use it in v1 applications.