Combination of textfields and checkboxes in a custom form
I ended up using a '#type' =&gt; 'container' instead of fieldset. And I used #prefix and #suffix in the children to add specific classes for spans and horizontal alignment etc. 

One other thing that seemed useful but I ended up not using: template files for form theming. This is a very useful tutorial: 

http://themery.com/dgd7/advanced-theming/forms/with-templates