I don't know if there's documentation, but how I consume flex licenses is by going to the traditional license portal (cisco.com/go/license), select the smart account/virtual account, then the PAKs are already loaded in the list for me, so I just have to find the right one (filtering on SKU contains flex helps), then after I've identified the PAK I need, I check the box next to it, then select Get License From Selected PAK, and that's it. It's just like non-flex licenses at that point. Put in your PLM or License MAC and download/email the license.
As far as mixing license types: Premium and Standard, I do not believe Flex changes how UCCX operates, and so no, you would not be able to mix them on the same UCCX.
