Remember that xlights was created to assist in programming thousands of channels needed for RGB lights. AC light support is pretty much an afterthought and some things have been added to make it easier on AC channels, but supporting AC channels is not what this software was designed to do and AC channels, while not going away are, in my opinion, a dying breed. I never found it easy to make my layout back in my LOR and then vixen days either.
Before I went all RGB, I did exactly what the others have suggested - made multiple models (painful) and finally just cheated and made a single model calling it a 4 channel RGBW string knowing that the colors would not look right on the preview but it would at least show my lighting timing and that was the most important issue. The layout preview is primarily to assist you in doing sequencing and for positioning props for whole house type effects. Just give up the idea that it has to be a true preview and you will be a lot happier IMHO.