Not sure if this is because it's in beta or if its broken. I'm unable to chose custom liveries to put into the formation editor, another issue is that the 43's will show up as full sets rather than the individual cars.
Do you have any examples? Only I can get many custom liveries in to the game (both my own and other creators)? The HSTs definitely don't work properly and, probably, fall in to the "multiple units in general aren't great at the moment" category. Here is an example of a CL66 community livery in FD (and some of my tankers behind it). Of course it might just be that I struck lucky with the combination that I selected.