So... I had to deal with a site that was original based on 16 cols, but, ended up with a few pages that were a mixture of 16 and 12 col layouts...

custom-grid.scss

//Bootstrap 3 grid inception

@mixin new-grid($class, $columns, $grid-gutter-width: $grid-gutter-width){
	$i: 1;
	$grid-columns: $columns;
	@include make-grid-columns($i: 1, $list: ".col-#{$class}-xs-#{$i}, .col-#{$class}-sm-#{$i}, .col-#{$class}-md-#{$i}, .col-#{$class}-lg-#{$i}");
	@include make-grid(#{$class}-xs);
	@media (min-width: $screen-sm-min) {
		@include make-grid(#{$class}-sm);
	}
	@media (min-width: $screen-md-min) {
		@include make-grid(#{$class}-md);
	}
	@media (min-width: $screen-lg-min) {
		@include make-grid(#{$class}-lg);
	}
	[class*=col-#{$class}]{
		padding-left: $grid-gutter-width/2;
		padding-right: $grid-gutter-width/2;
	}
}

// Usage
@include new-grid('12', 12);

index.html

<!---
	Markup Example
	- Base grid was 16, but, needed 12 column
	  layouts for a couple of pages that decided to 
	  have a 3 column layout...
--->

<!--- Original 16 col with 4 cols... --->

<div class="row">
	<div class="col-md-4">col 1</div>
	<div class="col-md-4">col 2</div>
	<div class="col-md-4">col 3</div>
	<div class="col-md-4">col 3</div>
</div>

<!--- New 12 col grid classes, for 3 cols... --->

<div class="row">
	<div class="col-12-md-4">col 1</div>
	<div class="col-12-md-4">col 2</div>
	<div class="col-12-md-4">col 3</div>
</div>