Use an html wrapper for your quantity and unit fields:
<%= form.input :quantity, :wrapper_html => { :class => 'fl' } %>
<%= form.input :unit, :wrapper_html => { :class => 'fl' } %>
And in your css (probably formtastic-changes.css), a css float property to float the li. I've added a bit of right margin to give the unit label some room to breathe:
form.formtastic fieldset ol li.fl {float: left; margin-right: 2em;}
You may have to have specific classes or ids for the wrapper_html and fiddle with the css to get your layout and spacing working the way you'd like.
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