I have the following code, the button is shown but the dropdownlist is not visible. I think the problem is with templatealias, but I dont understand very well how this thing works
I've tested your XML - you've used the group size "LargeLarge" from the Flexible2 template that dropdown isn't capable to support:
I've changed the MaxSize to MediumMedium (if you check the Flexible2 template in CMDUI.xml - it contains three rows of the small controls) and all have rendered correctly:
So you need to use as the scaling to your dropdown group any scaling that contains modes supported by dropdown.
BTW here's a recomendation for using templates in your custom groups:
UPDATE: Here's the walkthrough that includes a group template creation. I'm planning to cover this things in deep details myself but need some time to do it. :)