[Lombra]

Sure, there should not be any problems doing this. When tampering with the default UI you have to be careful not to taint sensitive stuff, but I don't know if the collection UI is in the danger zone.

You can find the collection UI source in the Blizzard_Collections folder in your extracted BlizzardInterfaceCode. Everything in that folder is exactly what the default UI is.

XML is used for structure (well, technically you could use OnLoad scripts to do everything), while Lua is used for functionality. You can, however, use Lua for structure as well, and most people do due to the excessive verbosity of XML.

There will be templates for the actual tabs, so you don't have to find all the right textures and stuff manually.

First try to find the template that's used for the collection tabs (look in the previously mentioned folder), check how they're created and positioned, and look up their OnClick script to see what happens when you click on them, then try to copy that and replace stuff with your own.