I then tried the following:
	Code:
	<customUI  
    xmlns="http://schemas.microsoft.com/office/2009/07/customui" 
    xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui" >
    <ribbon >
        <tabs >
            <tab id="newTab" label="New Tab">
                <group id="newGroup" label="Highlight">
                    <gallery idMso="TextHighlightColorPicker" showLabel="false" />
                </group >
            </tab >
        </tabs >
    </ribbon >
</customUI >
 That displayed the ribbon tab(after the error message), with an original size button in the group. 
Like your original try, there was no left/right dichotomy; both sides ended up displaying the drop-down gallery.
Also tried:
	Code:
	<customUI  
    xmlns="http://schemas.microsoft.com/office/2009/07/customui" 
    xmlns:mso="http://schemas.microsoft.com/office/2009/07/customui" >
    <ribbon >
        <tabs >
            <tab id="newTab" label="New Tab">
                <group id="newGroup" >
                    <mso:gallery idQ="mso:TextHighlightColorPicker" 
                        showInRibbon="false" 
                        size="normal"
                        visible="true"/>
                </group >
            </tab >
        </tabs >
    </ribbon >
</customUI >
  Same result.
I tried adding the split button control using the UI and ended up with something that worked fine. 
00 deleteme 1.png