This is because your "fixed-span" class is being overridden by the bootstrap span classes. You can solve this removing the word "span" from your class:
<div class="row-fluid">
<div class="fixed blue cols">...</div>
<div class="span8 red cols">...</div>
</div>
.fixed {
width:200px;
float: left;
}
Here's the fiddle: http://jsfiddle.net/KAfMB/4/