Use the html wrapper for quantity and unit fields:
<%= form.input :quantity, :wrapper_html => { :class => 'fl' } %>
<%= form.input :unit, :wrapper_html => { :class => 'fl' } %>
css (, formtastic-changes.css), float css float li. , :
form.formtastic fieldset ol li.fl {float: left; margin-right: 2em;}
, wrapper_html css, , .