The float: center;
won’t work because that element doesn’t exist in css. As you’ve noticed, only left and right will work for css floats – http://www.w3schools.com/css/css_float.asp
However, you could tweak your code and override the plugins css by doing adding this to your style.css file –
.social-icons {
float: none !important;
display: inline !important;
}
When I add the above code it centers all of the icons.